Deadhold is a gore infused viking inspired online game with a focus on tactical fast-action RTS gameplay. Control squads of units and explore the world of Deadhold with your friends in a raid-style cooperative adventure or compete head to head in Deadhold's competitive multiplayer.

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 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!

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.

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.