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.
The "download" link to left will take you to the download page on the Battle for Wesnoth site, where you can get the full, free game for your OS.
Oh, yeah... and as a sort of desultory handwave in the direction of actually trying to sell you stuff, if you like this, you'll probably like Styrateg.
The developer says:
The Battle for Wesnoth is a turn-based strategy game with a fantasy theme.
Build up a great army, gradually turning raw recruits into hardened veterans. In later games, recall your toughest warriors and form a deadly host against whom none can stand! Choose units from a large pool of specialists, and hand-pick a force with the right strengths to fight well on different terrains against all manner of opposition.
Wesnoth has many different sagas waiting to be played out. Fight to regain the throne of Wesnoth, of which you are the legitimate heir... step into the boots of a young officer sent to guard a not-so-sleepy frontier outpost... vanquish a horde of undead warriors unleashed by a foul necromancer, who also happens to have taken your brother hostage... guide a band of elvish survivors in an epic quest to find a new home.
200+ unit types. 16 races. 6 major factions. Hundreds of years of history. The world of Wesnoth is absolutely huge and limited only by your creativity - make your own custom units, compose your own maps, and write your own scenarios or even full-blown campaigns. You can also challenge up to 8 friends - or strangers - and fight in epic multi-player fantasy battles.
Features
- Build up a formidable fighting force, starting from a single leader and a small amount of gold.
- Over 200 unit types in six major factions, all with distinctive abilities, weapons and spells.
- Experienced units gain powerful new abilities as they advance.
- Several multi-player options available, including internet play.
- Scores of different custom-designed maps, and unlimited random maps.
- Hundreds of campaign scenarios available for download via a simple in-game procedure.
- 'Fog of war' feature available for a true test of generalship.
- Sophisticated mark-up language lets advanced users make their own maps, factions or campaigns.
- Excellent language support – 20 different languages currently available.
- GNU/Linux, Windows, MacOSX, BeOS, Solaris, FreeBSD, OpenBSD, NetBSD, AmigaOS4, OS/2 & eComStation compatible.
Reviews
"Overall, Battle for Wesnoth is a title that can at times be frustratingly hard, but is very rewarding at the same time. There is a massive load of content and multiplayer functionality and the game is a spot-on accurate piece of retro-gaming. And for the love of Shigeru Miyamoto, it's free people. What are you waiting for?... 8 out of 10."
-- Inside Mac Games
"The Battle for Wesnoth is a game that every strategy fan should play at least once."
-- Game Tunnel
"Playing kept me entertained for hours in both campaign and multiplayer mode."
-- PC Burn