So, aside from Minecraft and Dwarf Fortress, I can't find too many good looking Linux games. I'm running Fedora 18, with the Bumblebee Nvidia drivers installed, if that matters. Any suggestions are welcome - thanks!
Don't know if you're into that thing, but in my linux enthusiast club, we play a lot of red eclipse. It's cube based, it's like unreal tournament, single and multiplayer, fully customisable, and there are a lot of maps available. It's basic, but the graphics, depending on the map, are OK, and the gameplay is definitely fun, it's fast paced full chaos fps mayhem, more structured than TF2, with a futuristic theme, and besides guns it also has swords, which is a bonus.
What I play is: red eclipse and other cube based games, but red eclipse is my favorite, the doom series games, and then with steam I play TF2 and Trine2 mostly. Trine2 is great for kids as well as for adolescents and adults, and the graphics are great. I also play quite a few GOG titles in wine with PlayOnLinux, which is basically a collection of preconfigured wine-profiles for a long list of windows games, and older windows games in wine work quite well on modern linux boxes. Also fairly modern games often run well in wine, check playonlinux for a list.
PlayOnLinux isn't in the Fedora repos, but it's easy to install, RPM's available.
Sauerbraten - More interesting to let people google it and see what they come up with... Not really good looking but still entertaining.