THQ have just announced (instead of sending me the email they promised) that Space Marine has received an update adding a co-op mode. Called Exterminatus, because there wasn't a sillier word, if you load Steam you should find it updates automagically with the new option. (And you can laugh at 360 players, who don't get this yet.)
Exterminatus follows the recent trend for co-op modes, of rather than letting two people play through the game's campaign, instead it's a score-based endless attack from waves of enemies, with up to four players working together. I'm told you can play as the Tactical Marine, Devastator, or Assault Marine, along with all their weapons and perks.
There are two "scenarios", Assault On Hab Center Andreas, and Escape From Kalkys Facility, both of which will have a global leaderboard for competitive willy-waving. For the maximum wave, you can also add score modifiers, and complete "dynamic challenges", which will increase the game's difficult. And any experience you gain throughout all this counts toward your regular multiplayer ranking, because that's just the way it is.
Did I mention this was free? It's free. That's good.