I'd agree with Brett, and also add to check out hotwire.com. For some reason, the last few times we've been in Hawaii, the rental car rates were really pricey, $100+/day. But on Hotwire we were able to get it for $17/day for the same dates. I was blown away at the savings. 

You a member of Costco?  Go to costcotravel.com and they usually have great rates.

There are some smaller rental companies like http://www.mauicruisers.net/  You can Google "rent a wreck maui" to find additional off-market companies. I haven't used any of them but there are other options besides the expensive big-name companies at OGG...

Enterprise Rent-A-Car is a good central location just North of Lahaina.  We got a good deal on a jeep wrangler, but try to rent ahead of time as they get booked up quickly!  They even have a shuttle that will pick you up and take you back to your hotel.  The shuttle driver is very friendly and gave us advice on great local eats!!!  And don't forget to drive the road to Hana!  It's TOTALLY worth it!

I booked mine on Kayak.com, guess 2x National and 1x Budget were cheapest.

As usual, there's a number car rental company stands right outside the Kahului Airport (OGG) and you can just talk to them for the best rate. Even though they're all right next to each, their rates varied quite a lot when I was there about a week ago in March and you can use that to your advantage. I ended up getting a Jeep Wranger for about $275/week from Avis, which was cheaper than anything I'd seen online for that particular car.

Try http://www.discounthawaiicarrental.com

or http://www.mauicruisers.net

I would recommend you to use car rental comparison websites. They usually offer lower prices than car rental suppliers. For instance, Rental24H Maui provides car rental deals from Dollar and Avis. The prices start from $34 per day.

If you are looking for local companies and want to avoid the national brands, here are three. You can check reviews on Yelp too.

http://www.mauicruisers.net/

https://kimosrentacar.com/

http://www.kiheirentacar.com/

I would avoid Budget - I have heard more than once people waiting at least 45 mins to get to their car with a pre-existing reservation.