Trine
Trine recently went on sale, fulfilling my requirements for a purchase. Doing the whole co-op setup is kind of a pain - three Xbox 360 controllers and a Windows adapter for them, plus finding out that the Windows 7 RC doesn't support multiple monitors! Also there were some issues with Steam Cloud game saves yesterday, so we accidentally erased a few levels of progress figuring that one out.
It's way fun, though. Although the means are somewhat different, Trine encourages the same kind of co-competitiveness that Four Swords Adventures thrived on; death is a fairly trivial thing in co-op Trine (everyone resurrects at a checkpoint), so I have no reservations about throwing my teammates into fatal danger. As the Wizard, moving blocks around and dropping them on people is fucking awesome.
The game is architected such that, in single play, you can get through every level with each character, though some tasks are much harder playing as one or another. The real challenge comes when you try to get all the extras in the level - experience pickups and treasure chests, that expand each character's set of abilities. Now I can conjure platforms, the Thief can shoot fire arrows, and the Warrior can pick up and throw blocks! Neat.
So far I've played through the non-demo portions of the game exclusively in co-op, although I'd love to try it by myself sometime just to see how solo play goes.
Progress: Bramblestoke Village