Adventure

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.

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.

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.

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.