Not infrequently, you run into somebody posting about whether or not open-source development can possibly work for games, and usually concluding that it can't. Very likely the poster has played NetHack, but I guess Rogue-likes don't count. But. What about Battle for Wesnoth?
Battle for Wesnoth is a turn-based fantasy game in which you control a set of heroes and armies, building up over time to defeat AI-controlled opponents. A slew of campaign and scenarios in the game itself provide probably hundreds of hours of gameplay, but an active community provides innumerable new mods and campaigns you can download. It's been localized for something like 20 languages, and ported to just about every viable OS still in active use. And it is, of course, utterly free, both in the "free like beer" and "free like freedom" senses; the source code is open and available.
If you like turn-based fantasy games, you need to check it out; no, it doesn't have the amazing graphics of some commercial titles, but it's amazingly polished for a wholly community effort.