Orcs Must Die! [Steam store page]
While not co-op like the sequel, this game has an excellent sense of humor and decently balanced maps. Too bad the humor didn't carry over to Orcs Must Die! 2

Orcs Must Die! 2 [Steam store page]
They added co-op, but lost some of the humor that made Orcs Must Die! so fun. Similar gameplay, with more expansive levels to challenge two players.

Team Fortress 2 [Steam store page]
I wanted to love this game, but found it hard to get into because it requires team effort (mainly a healer at your back) or you die exceptionally fast. Unlike Unreal Tournament or Counter-Strike: Source where you can easily play a quick lone wolf match