Open source software games are not altogether common. Good game design, like good graphical user interface design, is a lot harder than it may appear; as a result, the majority of open source games that do exist tend to be derivatives or copies of commercial computer games. This is not necessarily a bad thing -- by opening up the source code, developers can experiment with alternative rule sets and graphics, while still giving players a familiar experience. That's why I often lament the lack of openness in simulation games: I want to be able to go in and tweak the underlying assumptions.
FreeCiv is a free/open source software version of the well-regarded "Civilization" computer game series. FreeCiv 2.0 was released this last week, running on Debian Linux, Windows XP, and Mac OSX, with ports coming for a wide array of other platforms. For those familiar with the commercial Civilization games, FreeCiv comes closest to the version of Civ from the late 1990s, Civ II, at least in terms of rules and graphics. It does have an outstanding multiplayer option, however -- unlike the commercial Civ games, FreeCiv was clearly built with a focus on multiplayer gaming.
[Civilization is a game covering no less than the history of human society. Starting as a nomadic tribe, the player starts building cities, researching technologies (starting with "writing" and "bronze-making," working up to "space flight" and "biotechnology"), and exploring the world. As you might expect, for me it's enormously addictive; it's one of those games I refer to as a "3am game" -- I start playing it in the evening, and before I know it, it's three in the morning.]
Surprisingly, the current commercial Civ version, Civilization III, is more readily customizable than FreeCiv. Civ III ships with a map and ruleset editor allowing would-be world-builders to make a substantial number of changes to the game's settings. Some developers have taken advantage of this ability to modify the game in order to come up with enormously more detailed versions of the basic Civ game. Rise and Rule, for example, adds hundreds of new intermediate technologies, culturally-linked units, resources, government types, city elements, and wonders; if you've become tired of the standard Civ III experience, I would encourage you to check it out.
But if you want to make changes to the rules beyond those allowed by the editor, you're out of luck. That's the value of the open source aspect of FreeCiv -- if you're willing to get your hands dirty with coding, you can go in and change any aspect of the game. There's a long list of modification projects. As volunteer efforts, of course, they're updated slowly, occasionally abandoned, and need additional people on the teams. One that I find particularly interesting is the attempt to introduce some of the rules and ideas from Alpha Centauri to the Civ game; AC was one of the better non-Civ world-building games out there, with environmental and diplomatic options I've not seen anywhere else.
Although I remain a big fan of the commercial Civ series, I'll continue to play around with FreeCiv, and keep an eye on the various mod projects. FreeCiv may seem a bit behind the commercial version in terms of look and feel, but it has a much greater potential as a game platform. If a "SimWorldChanging" is ever to get off the ground, FreeCiv would probably be a good base to work from. Although "reinventing the wheel" is something of the point of the Civ game, it's less attractive as a development path.