Steam

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!

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.

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!

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!

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.

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...

A mix of roguelike and classic hack and slash 2d dungeon crawling gameplay, Hammerwatch is the complete package that features single player, cooperative multiplayer online or on a LAN, as well as a level editor-- and it runs on Windows, OSX and Linux and allows for cross-platform play. 
Title is available both on GOG and Steam. Steam has a 4-player pack for $24, but GOG sells each individual copy for $4 right now, so that seems like a better deal.
DRM-free!

Released on Early Access in December 2013, this first person action adventure RPG  features cross-platform multiplayer on Windows, OSX and Linux. The game looks to be pretty early in development, but seems to be snaring enough people to invest the hours in playing it.

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:

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.

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