Know any good Open source games? I'm trying out some of these free GPL games, and my favorite is FlightGear. Open source shooters that does not say Cube would be nice.

A List!
I much enjoy Angband and it's variants. FreeCiv was pretty decent too.

Tremulous is a very good AVP-like deathmatch game made with the Quake 3 engine. It's one of my favourites
Spring is an excellent RTS; basically a 3-dimensional take on Total Annihilation.
Exult is a completely open-source implementation of Ultima 7, but doesn't include the data files you will still need the original game to play. If you like Ultima 7, it's pretty much the exact same thing.
Edit: Ois pointed out rougelikes. Slash'EM is the one I play the most. It's a very challenging turn-based dungeon-crawl.

I don't want a list, I want personal opinions. If I wanted a list, I'd go to Ye Olde Google.

I did mention two others I enjoy :)
And somebody may want a list. Possibly. No loss if not.

Warsow is a fine balanced shooter, which I enjoyed playing. I don't play it a lot anymore.
I think it's still used in tournaments.
If you wanna check it out go to www.warsow.net

lbreakout2
at least if you like arkanoid like games