I used to like Monday Night Combat, but absolutely no one plays it anymore. Was a cool mix of tower defense + fps.
I played MNC on the 360, so I can't comment on how it was for the PC. I thought that unlike TF2 a lot of the maps were very complicated, so you would get snipers going to places that required 2+ jump pads to get to, sniping nearly uninterrupted for the whole game, which caused newer players to leave out of frustration. The engineer's healing gun was a bit underpowered as well. Overall, it did have a great concept though.
Oh uh, steam games? I pretty much only play TF2, but my friends have been forcing me into Dungeon Defenders and L4D2. They're alright.