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.
I've been playing the
I've been playing the development branch (on a Mac) for a week or so. I enjoy the game: it's got some nice features and some good playable campaigns. In addition to being free and fun, they've got some good ideas. The use of veteran units that can be recalled from one scenario to the next adds an interesting dimension to the game: in addition to defeating enemies, you're also carefully cultivating the units under your command. This can make for excruciating decisions when playing scenarios where the objective is merely to survive a length of time. The use of day and night is also interesting; if you choose to use it, it can be quite helpful.
The included campaigns are hit-and-miss. I've played through three of the beginner campaigns so far, one of which (Heir to the Throne) is quite long. All of them have been playable, but the writing quality varies. The South Guard has been my favorite so far. It offers the player real decisions which greatly change the way the campaign plays out -- there isn't a wrong answer, either: whatever you choose, the result is interesting and fun.