I just played Xonotic for the first time ever and now I'm interested. I like a game I can be sure isn't selling my data to China.
I would personally like an RTS, a game in whatever genre Stick Fight falls under, and sandboxes, although any genre is fine. I don't contribute code, but I'm pretty good at finding bugs, etc. and get a few laughs out of people on the forums with my jokes.
Games in early development preferable, mostly for the active community these tend to have, so I can share less formal work with them and join their Matrix.org room or something
The only open source games that come to mind are OpenTTD (a clone of Transport Tycoon Deluxe), FreeCiv (clone of Civilization) and FreeCol (clone of Colonization).