Hi Ekimino,
Off the top of my head a few successful open source projects you may want to spend time digging through and engaging with their communities are:

• Audacity
  (There's a good case study on it's development in The Art of Unix Programming: Case Study: Audacity)
• VLC

I'm sure you'll even find interesting ways you can contribute which would give you great opportunities to learn.

Good luck and keep us updated once you get going.