For me Battle for Wesnoth is like being beat in the face by a large plank with nails driven in it. On any difficulty level above "drooling idiot" I get roundly trounced.

Come to think of it, I don't believe I beat FF:T either. There may be a pattern forming here...

Oh, and my favorite free to play game is Minecraft. That's still free to play even though Notch has entered beta, right?

[[Edited to add, well, relevance]]