Third Person

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!

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.

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 3D shooter that pits the pilot of a ship defending the shafts of mines against devious bots. The game's unique selling point was the "six degrees of freedom" that allowed the player to move freely along all three axes in the game world.
Both Descent and Descent II are available from GOG for Windows and OS X; there is also a source port called DXX Rebirth.

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.

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.