Strategy

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.

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!

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).

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 heist game isn't quite GTA but it is a cooperative bonanza: it works on Windows, Linux and OSX and supports both network and local cooperative play as well as cross-platform multiplayer. PC gamer in specific called out its excellent cooperative play mode.
Plus, it's on sale on Steam right now!
In fact, I think Blackstar and I are going to try this out at some point. I may even have some extra codes if there's interest...

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.

Out now on Steam on Early Access, Entropy is listed as a massively online action strategy RPG. Sounds a bit ambitious, no? And to boot, cross-platform multiplayer on Windows, OSX and Linux. (I'm wondering if the phrase "Unity Engine" is in there somewhere... it does look like the studio has used it in the past.)
Here's what the devs say:

Would you like to play a game?
Introversion's recreation of Global Thermonucler War from the 80s classic film WarGames has competitive and cooperative multiplayer modes against humans or bots, and supports crossplatform play.

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.

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