Aside from the ones already mentioned, some apps I really love are Trover and Findery to read about first hand account of places that I plan to visit from fellow travellers. I also love using Foursquare everywhere I go, to look for ratings and reviews locals give to the places they've been to and there are great deals there too. Triposo also is a nice app to get complete guides on cities that I would go to. It certainly beats buying those travel guides from a bookstore or the airport. Another app I love is TouristEye, it aids in trip planning with suggestions for popular local tourist spots then giving you an itinerary that's available offline. For road trips, Roadtrippers is amazing because you just determine your start and end locations and it gives you places to see along the way. Field Trip also works similarly but with this app, you get notifications when you get near the location. 

Flight Booking / Researching:

ITA is my favorite resource for searching for air fares.  When I am in Europe and need to search for LCCs as well, I use SkyScanner

Hotel Booking / Researching:

For Hotels, Kayak is my favorite website since it aggregates everything.  I  lean towards Booking.com in Europe since they have more B&B coverage, which is especially useful in small towns where chain hotels just don't exist. 

Pre-Destination Booking / Researching:

In most cases, I rarely have enough time to plan everything out in advance so I usually quickly hop on Oyster to see if there are any books on the destination.  I download all the eBooks onto my iPhone on the way to the airport to skim through the flight. 

At-Destination Booking / Researching:

When I am at the destination, I usually hop onto Wikitravel and also to Viator or Peek to see if there is anything interesting. 

I typically start my trip planning by generating a list of places to see and things to do. That list is usually kept in a Google doc. Then I put the items onto a Google map so I see where they are in relation to each other. After that I try to figure out which places make sense to see together on the same day. I'll usually have a loose schedule in my head for what I want to do each day. 

I have started using Pinterest to pin items to a map. I haven't used it yet on a new trip so I can't say how well it works yet. 

Before I arrive I'll load my trip details to Tripit so I have all my flight and hotel information in one place. If I need a place to stay I'll use Airbnb or Booking. 

On the ground I'll use Google maps to find my way and for transit directions. I'll also use Yelp or Foursquare, but that's usually when I'm looking for some place close by for a drink or a bite. 

There's an awesome translation app called Word Lens.  It's amazing.  You hold up your phone in live camera view and it overlays a translation of signs, menus, etc.  You can then freeze the translation.  There are multiple languages available.

I keep going back to www.onebag.com for tips and tricks - their bundle wrapping technique is a firm favourite

Aside from the obvious necessity apps that I use for both person and work travel (i.e.,, United, Hilton)...I adore the apps like flysmart (gives you airport information like parking availability, shopping stores and restaurants with reviews for tons of airports), afar (can search locations and pics with brief descriptions pull up), loungebuddy (tells me where airport lounges are, and if my enrollment gets me in for free or my status with an airline gets me in, or what cost to use), travelzoo (deals!), uber (awesome), seatguru (fabulous for long flights!), and for fun I like to peak at tvfoodmaps/foodnetwork on the road!

1. I always use TripIt when traveling, keeps all my transportation, meetings, etc organized in a nice clean way.

2. I often use Foursquare/Swarm as well when I want to find something interesting near me.

3. As someone else mentioned, Word Lens is pretty helpful when you don't speak the language. 

Here are my top travel apps:

1. Pocket Earth : Ever since I discovered this map app, I have never traveled without it on my smartphone. I download all the city maps I need beforehand, plot out the places I want to visit, and then just pull it up from my phone when I need it. It has never failed me and has always given me accurate directions. 
2. XE Currency : I love shopping so I need a quick currency converter before I spend. 
   
3. Booking.com : Always my go-to site for my hotel/ hostel bookings
   
4. A train/subway route planner app : I love taking the trains/ metro when I travel so I always download a route planner app. Recently, I've used the map apps from Urban-Map . I got the Tokyo, Osaka, and Seoul rail map/ route planner apps and they were awesome. 

I love all the info your question has generated, and I'll definitely be checking out the suggested apps, but my tip is about hardware: I take my iPhone and mini-iPad, with a cable to upload my pix from phone to pad at the end of each day. In many hotels, the WiFi connection is not strong enough for me to accomplish this task wirelessly in brief timeframe, so the cable makes it simple and fast.  I then can edit pix in a larger format, post the best ones, and free up the memory on my phone for next day's photography.

Travel Tracker Pro - only app I found that lets you plan private car road trips a well as commercial

Tripit

Packing Po

Hipmunk

My #1 is XE Currency. I'm not great at math or off-hand conversions so it's really helpful especially when visiting multiple countries with differing currencies in a short timespan and you're trying to stay on a budget.

TripAdvisor is an old standby for accommodation and activity reviews, Yelp for restaurants reviews. Google maps for getting around, street views to find locations and updates of traffic. 

Other apps I use for travelling are just my usually day to day apps helpful for regular activities, my calendar, reminders, banking, notepad and for longer trips Skype to talk to friends and family without paying roaming cell phone charges. 

More then anything though I use my phone as my quick point and shoot camera to capture travel moments without having to grab for my more expensive camera usually out of it's travel bag. 

Some of my favorite travel tools:

• Tripcipe, a new travel startup (still in private beta) helps people easily save recommended sights or eats from any website--it's like a Pinterest for travel. It's great for people who do a lot of planning and research on multiple websites; Tripcipe keeps all the information in one place and offers a clean interface while showing everything on a map. There's also a mobile app coming soon (with offline features) so that you can take all your research with you, even if you're abroad without a mobile plan. To see an example, check out this New York Itinerary on Tripcipe. (For full disclosure, I'm one of the co-founders of Tripcipe. ;) We designed our site to make trip planning as easy as possible from start to finish.)

• Google Maps App, which now offers offline maps.  To download a map offline, first go to the area of the map you want to download (for example, the city of Paris).  Then type in the search bar on top: "ok maps".  The app will download the map area on your screen; you'll be able to zoom in down to the street level.  Infinitely helpful when you're abroad without a data plan!

• Design Sponge City Guides.  This isn't a tool per se, but it's one of my trusted sources for any city.  They offer highly curated recommendations that range from food, sights, to shopping... all with a design spin.  I love that their city guides will generally cover the "big sights" but also call out more eclectic and unique finds.

Other apps/tools I use have already been mentioned by others, such as Evernote (handy for taking full articles offline) and Tripit (great for organizing logistics).

I would suggest you carry a list of the private guide & group tour descriptions offered by the following companies.These can often offer an alternative to the guide books and help itinerary planning as well -whether you want a private guide or not.

http://www.Toursbylocals.com

http://www.anyroad.com

http://www.tourdigger.com

As well I often have a look at http://www.i-escape.com for their background material on a destination I am traveling to.

Best 

Zel 

compare hotel prices and get always the cheapest price for your hotel

www.trivago.com (also has android app)

find out not were tourist go but genuine places from locals

www.spottedbylocals.com (android app)

website also sells small guides 

I've alay loved guide books, to prepare planning beforehand and on site for eveyday use.  Now that they're elctronic it's even better. Great with my iPad.

i also like google maps, when on the road, but you need to be online most of the time.  My tip is to take a pictor of the map by pressing both bottons (front and top on/ off). It stores it in photo portfolio you can use whenever.

My favorite app is Paking pro.  It help you on planning packing, things to do beor leaving nd alo is a great recco of what you bring along if luggage get lost or stolen.

Hipmunk for flights.

TripAdvisor for reviews

Zip lock bags for packing your lunch at breakfast.  Food is very expensive.  Small Thermos flask and coffee so you can take your coffee along.  I like to take the cupochino bags that just need sugar and hot water which you can get at breakfast.

I like using Hipmunk to help plan my trips, then I use TripCase to keep track of all my information!!!

I'm with Scott, Word Lens is so helpful and makes life so much easier.  Then places like here, Twitter and Trover to see what's around you

Trip It: www.tripit.com

Google Drive: https://drive.google.com

Like others here I enjoy using Kayak and Skyscanner - great for planning routes. I also use TripAdvisor, Yelp and Urbanspoon to check out restaurants/bars. There are some great city specific apps - a Shanghai taxi translator app was brilliant. You type in the address in English and show the flashcard to the driver. I'm also a big fan of metro/subway apps for each city. This was particularly useful in Paris when stations can close at odd times or weekends, to be able to devise a 'work around'. 

All of my favorite travel tools are listed for free at:

http://www.worldleap.co/resources/

- I hope that helps!

Tripit Pro - keeps getting better and better!

Peek.com - just came upon this tool.  Beautifully organized.

Word lens (great for menus and maps) and skyscanner....essential.

I really like using Wander - which is a site that allows you to search for travel destinations that match your budget. it saves me a lot of time in jumping back & forth between sites just to "build" the custom package.

My travel apps of choice are:

Maps.me for offline maps (http://maps.me/en/home)

Atameo as a travel journal/blog/gallery (https://www.atameo.com)

Hyperlapse vor stable video (https://hyperlapse.instagram.com/)

Komoot to plan hiking tours (https://www.komoot.com)

Evernote to keep notes and tickets in one place (https://www.evernote.com)

Just come across Outbound app. Great way for meeting fellow travellers. Also has a useful wifi finder. http://www.outboundapp.org

other apps I have used a lot in the past: offline pages (save internet pages for viewing offline), timecave (set up emails to send at a specified future date/time eg set up an email to be sent to yourself in 2 months timeto remind you to send a birthday card to a family member), tubebox (for saving youtube videos to watch offline), world clock, currency, mapswithme (works via GPS offline and you can see where you are on a map), word lens, photocard (for sending physical post cards via iPad or iPhone), the favourite function on Dropbox for viewing documents offline (like flight confirmations etc).

Your Travel Agent!

Wikitravel does the most for me :) It really depends what kind of a traveller you are! The sources for the person who wants to party all the time is much more different from the nature lover :)

For trip organisation I use Tripit and Worldmate.

For Navigation abroad I use the Android app Locus and download OpenStreetMaps for offline navigation. Locus pro also features Turn by Turn navigation. 

An alternative to that is Nokia Here Maps, which also allows you to save maps offline.

A nice tool (if you have internet on your phone) is Google Field Trip. It also reports interesting things to you, if they are close by and you can set it up to your needs

If I'm trying to stick to a budget, I'll use Trail Wallet. https://itunes.apple.com/ca/app/trail-wallet-travel-budget/id547171665?mt=8

Otherwise, www.tripit.com to plan out the travel days when I have pre purchased flights/rail/ferries or activities. 

I love www.wikitravel.org for initial planning. I use google maps quite a bit as well as Facebook and Twitter when there to find out where everyone is and how to get there. 

I don't use apps, due to my travelling with an old brick rather than a smartphone ... but the websites I use to plot and research are:

Skyscanner - for sourcing air fares.  I don't necessarily buy them through there, especially if I think it'd be cheaper to go direct to the airline's website, but it's a very good site to look up and check.

Hostelworld - for seeking out cheap accommodation.  Again, I may end up not booking through there, and their coverage lacks in certain countries, but again it's a useful guide, and very good for last minute 'oh drat I've changed my plans' or late arrivals in cities.

Wikitravel - for learning about a place, including important things like how to get into/around a city, and what's there to do/see/eat/drink.  Easier than a guidebook and often more amusing.

Twitter - would you believe!  Useful for communicating with other travellers, especially in one of the legion of travel chats.

:)