Free To Play

Unturned is a free-to-play cross-platform action zombie survival game with crafting elements.

The successor to the Quake engine-based Nexuiz, Xonotic is a fast-paced multiplayer shooter, similar to Unreal Tournament or Quake 3 Arena. Features FFA and team play.

This modern port of Stuart Cheshire's classic tank multiplayer game Bolo still runs on modern versions of OSX, is still free, and is still fun! The game's homepage is down as of this writing, so we may even try to get permission to host this one ourselves. A map editor is also available.

According to its sourceforge page, Triple A is "a free online turn based strategy game and board game engine, similar to such board games as Axis & Allies or Risk." While it doesn't appear to have a discrete cooperative play mode, these kinds of games usually feature the ability for players to create informal (and usually temporary) alliances among themselves, so I'm including it here. Thanks to a former IMG writer who tipped us off to this gem!