Real Time

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!

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.

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.

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!

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.

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.

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. 

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.