When I had a regular static group (before massive illness), we played exclusively on steam. The first one was Left 4 Dead 2...highly recommend that game to anyone who enjoys the genre. It was a great find, and I personally enjoy the fact that Steam updates their games automatically upon login.
We also played Sanctum. I was the weak link here, as my guys were ranked and I was das noob. But for a "defend the keep" sorta game, it was hella fun. We built our own mazes on the platforms for the baddies to worm through, then positioned artillery and ourselves atop the maze.

Lots of fun, a bit of a learning curve, though.
I also played a DnD game on there, don't remember the name exactly. But it was lame, in a fun, DnD way.

Lots of laughter and poking fun at the horrible animations and attacks.