Battle For Wesnoth!
 

So in my endless pile of open source and free games that I've discovered, I think this will always be one of the best. If you played around in the Playstation 1, imagine the original Final Fantasy Tactics with a village capture system, more races (and traditional ones too), extremely modable, but with more set attacks per unit. Oh, and online multiplayer too.

Anyone play this before? I know it's been around for a few years.