I've played most of the FPS's on there, OpenArena is so similar to Q3 Arena that I don't miss Q3 much, I just fire that up when I want some of the action. Nexuiz is quite cool too, as is Alien Arena. World Of Padman is an insane amount of fun, but it can take some getting used to.
As for Tux Racer, its fun for five or ten minutes, but its nothing to keep you around beyond that. I've not played TORCS in years so I couldn't tell you much about that unfortunately.
Wesnoth gets a huge amount of press in the open source crowd, but I've not really played it much. I find it hard to get into RTS's that dont have an air of futurism about them, whether thats briefly ahead in time like C&C, or miles into the future like Total Annihilation.
Plasma Pong is trippy as all hell, it's not the most amazing game ever made but you feel like you've taken something when playing it.
I've not really played much else on there, but my experience so far has been that most of the FPSs are the best of the lot. A big contributor to this I feel is a lot of the commercial FPSs have open sourced their game engines (the Q3 engine is a good example) so the guys building these games can spend more time on tweaking and less time on just providing an open source alternative regardless of quality.