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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!

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!

This Kickstarted game will be released "when it's ready" but next year is as good a year as any for this sci fi real time strategy game. 
About the game:

Colonize solar systems, smash entire worlds, and obliterate your foes in epic battles with multiple players and thousands of units. Planetary Annihilation takes real-time strategy to a planetary scale. Control rumbling, powerful tanks of war, massive bomber jets, and an assortment of subs, bots, orbital units, and battleships.

In December 2013 the publisher confirmed that ESO will be coming to Mac OS X as well as to Windows. The expected release date is April 4, 2014. The MMO RPG based on the Elder Scrolls universe will use a subscription model, like World of Warcraft (also available on OS X). Exact specs aren't out yet; the systems we list here are guesses. Console versions for PS4 and Xbox One will follow in June 2014.

A re-creation of the original Half-Life in the 2007 version of the Source engine, with upgraded high definition audio and video-- up until the Xen chapter, at least. There's no native OSX version, but if you install Steam into a WINE wrapper, you can use it to install the 2007 Source SDK and play it on OSX.

Markus "Notch" Persson created this block-based game in 2009 using Java. Years later, versions exist for PC, OSX, iOS, Android, and Xbox, with a PlayStation version in the works. 

OpenRA is a modern crossplatform engine for the Command and Conquer series of RTS games. Supported games include the original C&C, Red Alert, and Dune 2000. This engine requires the original game data files to work. They were released as freeware but appear to be no longer available from EA following the announcement and subsequent cancellation of a new F2P C&C Game. The original C&C was first released in 1995.

0 A. D. is a real-time strategy game of ancient warfare where players build and develop their own city-state to compete with other human or computer-controlled city-states.

This prequel takes place 1,000 years before the first Myth game and tells the story of the hero of the Wolf Age, Connacht. Unlike Myth 1 and 2 which used sprites for units, Myth 3 uses polygonal models.

Sixty long years after the defeat of the Fallen Lords, Soulblighter and the forces of the Dark return once more, bringing an ancient terror with them. Playing Myth or Myth II requires an original disc. Demo files are available at ProjectMagma.The modern Myth II engine also supports a number of plugins and third party scenarios, also available at ProjectMagma.

Bungie's take on the real time strategy genre is actually a real time tactics game with resource management removed and a focus on combat tactics, successful use of terrain, and an epic fantasy story.

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