[2012 Update] - Traditional RPGs are non-existent, as publishers favor F2P-MMORPG. The Action RPG genera is way over-saturated, to the point, even the mighty Diablo took harsh criticism.
[2013 Update] - There just may be a "traditional" coop RPG in the works.
For a complete list of all PC coop games per year
For a list of the Best Non-MMO Multiplayer RPGs of all time
Divinity: Original Sin - 2013
Two Worlds 2 - 4 player - 2011
Can not coop single player campaign. Instead, coop campaign will take place between the first and second game.
Fable 3 - 2 player - 2011
Features drop in/drop out multiplay; you can carry over items and gold; plot progression does not transfer; enemies do not scale up; and something about marring your coop buddy...
Action RPGsThe Incredible Adventures Of Van Helsing - 4 player - 2013
Sacred 3 - 4 player - 2013
Blood Knights - 2 player - 2013
Marvel Heroes - 2013
Grim Dawn - 4 player - 2013
Dungeon Siege 3 - 4 player - 2011
Gas Powered Games will not be developing this next entry, but rather Obsidian. The DS series is best known for being a good Diablo clone, although DS2 fell a bit short in my book.
Magicka - 4 player - 2011
Diablo clone with just wizard types. Lots O'magic options. Satires overplayed ARPG genera.
Magicka: Vietnam - 4 player - 2011
Expansion to Magicka, only it has goblins with guns.
Torchlight 2 - 4 player - 2012
The first Torchlight was sortta the dev testing the waters to see how they can fare with a new IP in the ARPG market. With some success, they push forward with the ever excepted multiplay element.
Diablo 3 - 4 player - 2012
A continuation of the Diablo series. The original talent is no longer on the license, but the new team has a great resume.
Metacritic - 90
Free 2 Play:
Path of Exile - ARPG - 2012
Another Diablo clone, with more blood. It dabbles into MMO space, so I'll need more info on this title. Defiantly another Free-to-Play micro-transaction game.
Legends of Aethereus - 4 player - 2013
All Zombies Must Die! - ARPG - 4 player - 2012
Metacritic - 58
The First Templar - (FPS) 2 player - 2011
Lord of the Rings: War in the North - 3 player - 2011
Dungeons & Dragons: Daggerdale - 4 player - 2011
Metacritic - 49
Darkspore - 4 player - 2011
Metacritic - 65
Shooters with RPG elementsBorderlands - (FPS) 4 player - 2009
Borderlands 2 - (FPS) 4 player - 2012
Considered the fastest selling original title of 2009. Could it be ...people really still crave this genera, numbers don't lie.
Metacritic - 81 & 89
Dead Island - (FPS) 4 player - 2011
Dead Island: Riptide - FPS - 4 player - 2013
A cross between L4D and Borderlands. Game is nothing like its mind blowing trailer.
Metacritic - 80
Mass Effect 3 - (shooter) - 4 player - 2012
Fight alongside your friends as the galaxy goes to war. This mode will enhance the single play experience without directly effecting the storyline of single player.
For a complete list of coop games per year