The Capital One 360 Café is a really good cafe with free wifi. It even has a couple of conference rooms downstairs if you need to meet in private:

http://www.yelp.com/biz/capital-one-360-caf%C3%A9-san-francisco

If there are just a few of you guys, here are some of my favorite places:

• 901 Columbus Caffe in North Beach. The guys who work there are awesome! I used to practically live here with my laptop. Plenty of outlets hidden around the place and reliable wifi. If you're typing "Columbus Cafe" into your map, make sure you near to the one at Columbus St, not the bar of the same name on Green Street. I can guarantee you there's no wifi there, nor much lighting :P
  
• Ritual Coffee Roasters in the Mission is pretty cool. This is where coffee snobs go and every cup of coffee is a masterpiece. It might be hard to find a table during peak hours and there aren't too many outlets at the single tables, but there's an outlet strip at the communal tables in the back.
  
• Coffee Bar in Potrero Hill is the same situation as Ritual. Hard to find seating during peak hours, but there are communal table there.
  
• Another Cafe in Nob Hill is also the same situation as Ritual and Coffee Bar.
  
• Workshop Cafe is fairly new and is an actual coworking space in the Financial District. It's gorgeous. There's a free area towards the front, and a paid area towards the back, and it's super cheap. You can reserve a table through their website and it's worth it knowing that you'll have a guaranteed table and outlet. You can even order coffee from their website and they'll deliver it to your table once you're there.