Microsoft has launched their Games for Windows On Demand service, and despite initially claiming 5 titles, it appears the service is offering 13 at launch. Of the co-op variety we have Resident Evil 5 and Kane and Lynch: Dead Men.
Most games are listed at full retail price, but there's a free title as well as a few sales which I've marked in the list below. If you don't have the Games for Windows Live client, you can download it here.
Here's the full list:Fallout 3 DiRT 2 Resident Evil 5 Street Fighter IV Battlestations Pacific FUEL Viva Pinata Kane and Lynch: Dead Men - $19.99 Shadowrun - $9.99 Section 8 Juiced 2 Red Faction Guerilla Tinker - FREE!
I recommend Shadowrun whole heartedly for $10. And no, I'm not joking. Also - Tinker has achievements.