0 A.D. (pronounced “zero-ey-dee”) is a free, open-source, historical Real Time Strategy (RTS) game currently under development by Wildfire Games, a global group of volunteer game developers. As the leader of an ancient civilization, you must gather the resources you need to raise a military force and dominate your enemies.
It started its life as a closed source indie game about a decade ago, but has been fully open-sourced a few years later and the community has since then made great progress. Recently it also ran a relatively successful crowdfunding drive, which allows them to have a full time developer for about a year now.
0 A.D.'s art assets are available under the Creative Commons Attribution Share Alike (CC-by-SA 3.0) license and the source-code under the GPL v2 or later. This makes 0 A.D. a Free Software title.