Wesnoth is a turn based strategy game, with RPG like elements. I got immediately hooked when trying it out the first time, and that after expecting just another low quality unfinished attempt at a GPL game.
Something really remarkable also is the number of enthusiastic contributors Wesnoth has. Their music sounds like done by professional orchestra composers, and their pixel art is top notch. And they currently have the best available maintainer for their Python scripting (me) :)