Battle for Wesnoth is the Free TBS game, arguably the Free strategy game: the best known, the most played and probably the best looking. But beside it, there are other smaller, less known gems, like 8 Kingdoms, a game which got packaged for Fedora after the F8 release, so I believe it is not part of any spin or media release, but is only a yum install away...
Before the obligatory screenshot I feel I need to put a warning: the user interface sucks and the graphics are quite ugly, but I like its gameplay better than Wesnoth's and find it really addictive.
Surely, there are some other things I would like to see in such a game, like more depth, a single player campaign, a backstory and maybe some role playing elements and features which might come handy (like a movable camera angle, which one would expect from a 3D game). And less crashes.
But considering it started by some students as an university project and it has about a couple of active developers, it is really awesome. A must play. Maybe also a must contribute, too bad I know jack about 3D modeling.