Built from the ground up for cooperative play and supporting OSX, Linux, and Windows, Left 4 Dead 2 definitely belongs on this list. And for a short time during the holidays in 2013, Steam offers the game for free!

Narcogen shows Blackstar some of the upgrades he's made around the house. Then we take a wolf mining with us...

Game: 

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...

Blackstar and Narcogen have been sent back to primitive Earth by a freak teleporter accident. They must develop the technology to return to Mars, the construction site of the Marathon, to save the future!

 

In this episode, we look at two more of the installed mods: Elegant Punt, which adds new small boats to the game featuring storage and extra seats, and Somnia, which simulates the world while you sleep so that you can smelt ores faster. (Mobs still spawn, though.)

 

Small Boats (Elegant Punt)

http://www.minecraftforum.net/topic/1599100-wip-small-boats-elegant-punt...

 

Somnia

http://www.minecraftforum.net/topic/1938500-164forgesmp-somnia-simulatin...

 

(Nearly) Complete mod list:

http://osxcoopgames.net/content/build-marathon-minecraft-mods

 

 

Game: 

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.

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.

Build the Marathon Ep. 3: Galacticraft, NEI, Dimensional Anchors

 

Blackstar and Narcogen have been sent back to primitive Earth by a freak teleporter accident. They must develop the technology to return to Mars, the construction site of the Marathon, to save the future!

 

In this episode we look at a map to see where we need to go, check out Not Enough Items to find out how to build things like oxygen gear we'll need to survive in space, and dimensional anchors to keep ourselves from floating away into space (and keep wheat growing while we're away).

If you're looking through any of the coop games for OSX for sale at Good Old Games, note that from now on, they're offering a money-back guarantee.
http://www.gog.com/news/announcing_our_new_guarantee

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:

I hate to have to diverge from the stated focus so soon, but this looks fantastic. Reminds me of one of my favorite games on the Amiga, Starglider II. No idea whether it will support coop play, multiplatform play, or even run on OS X, but I don't care. I want it.
UPDATE: It seems like coop is unlikely, as recent interviews have quoted developers as saying "you'll never see another player" and while I applaud the emphasis on exploration and the removal of griefing that focus brings, it'd be nice if you could party up with one or two other players to explore the universe together.

I've opened this site to make a home for content that doesn't really belong on Rampancy.net, because of that site's focus on Bungie. So here's where I'm coming from. For the past several years I've been working on Bungie fansite Rampancy.net. Its major focus since its founding in 1999 as "The Core" was Bungie's sci fi shooter franchise, Halo. I became a Bungie fan in the 90s for several reasons: they made good games, and they made them for Macs, which was the platform I migrated to after the Commodore Amiga became unviable. But that's another story. When Jason Jones joined Steve Jobs on stage in New York during MacWorld in 1999, it seemed like a new age might be dawning for gaming on the Mac platform. After making the excellent Myth RTT games as crossplatform titles, Bungie was returning to sci fi with a new shooter game. It would be on Macs as well as PCs in 2000. Then everything changed. Microsoft bought Bungie; Bungie made Halo an Xbox exclusive, and Microsoft replaced the Xbox with the Xbox 360. I replaced my Xbox 360s when they broke (sometimes for free, sometimes not) so I could keep playing Halo. Bungie kept making Halo games-- even past the point where it seemed, at least to me, that they wanted to. Now independent from Microsoft, Bungie is making a new sci fi shooter franchise for current and next-generation consoles. I don't need to worry about what I'll be playing Destiny on, because there will be a 360 version. Perhaps I'll have a decision to make when the sequel comes out; Bungie is being cagey about a PC version of the game, to say nothing of a version for OSX. In the meantime, online play became not just a desired feature but a required one, and online coop was added to Bungie games new and old. Over at Rampancy.net, Blackstar and I are currently playing through the Marathon series in cooperative mode using the community-built open source Aleph One engine, despite being on opposite sides of the globe. It's a blast. I want more.

Blackstar and Narcogen have been sent back to primitive Earth by a freak teleporter accident. They must develop the technology to return to Mars, the construction site of the Marathon, to save the future!

 

The first two episodes are in unmodified "vanilla" Minecraft, version 1.6.4.

Game: 

Blackstar and Narcogen have been sent back to primitive Earth by a freak teleporter accident. They must develop the technology to return to Mars, the construction site of the Marathon, to save the future!

The first two episodes are in unmodified "vanilla" Minecraft, version 1.6.4.

Game: 

Pages