OSX 10.8

A procedural piracy game, you command a ship to sail the seven seas, either alone or with friends. Explore, do quests, fight pirates, or be one yourself!

First-person survival game where you're stranded naked and helpless on an island full of dinosaurs.

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

Interstellar Marines bills itself as an old school tactial FPS, and just recently added a solo and coop player campaign mode called the Neurogen Incident.

A turn-based fantasy RPG in the vein of Baldur's Gate featuring cooperative multiplayer as well as a single player campaign.

This procedurally-generated voxel open world game looks like Minecraft on shaders and steriods. It's being kickstarted later this spring, with an alpha expected this summer, and a release later this year for Windows and Mac OS X.It still seems sort of strange to me that developers look at Minecraft and think the way forward is better graphics, but hopefully there's also a good understanding of what makes Minecraft fun.

Proven Lands is a sci fi survival sandbox game with release planned in 2015 for Windows, Mac OSX, Linux, iOS and Android. Solo, multiplayer and cooperative modes are included, and it is currently being both Kickstarted and Greenlighted. Check out the video, this one may be worth looking into next year!

The collection is actually the first game in the AI War series, Fleet Command, and five expansions released between 2010 and 2013. The developer, Arcen Games, is working on a new entry in the series due out later this year, called The Last Federation. All feature competitive and cooperative play against human or AI players. It runs on Windows, Linux and Mac OSX, and supports cross-platform multiplayer.
Steam has just put up for sale the entire AI War Collection: six titles for less than five bucks!

This free, open-source empire-building game lets  you lead your tribe from the stone age to the space age. Now there's an HTML version playable in browsers, even on mobile devices! Features network and play-by-email options as well, and supports Windows, Linux and OSX with cross-platform play. Currently the OSX version is out of sync with the latest stable version (latest Mac version 2.3.0, latest stable for Win/Linux 2.3.5).

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.

The latest iteration of Sid Meier's famous strategy game series doesn't get listed on most sites as being a cooperative game, but that's only because how most alliances are temporary. That certainly doesn't need to stop you from getting together online with a few friends to beat up on the AI (or try to) before turning on each other!

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!

This squad-based first person shooter is a full release based on a Source engine mod; it runs on OSX and Windows and supports crossplatform multiplayer as well as competitive and cooperative maps.

Built from the ground up for cooperative play and supporting OSX, Linux, and Windows, Left 4 Dead 2 definitely belongs on this list. And for a short time during the holidays in 2013, Steam offers the game for free!

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