Specifically, the Mass Effect 3 demo, and multiplayer.
Now, I know lots of folks don't care about ME3 multi, but think of it as a free game mode with your purchase of ME3. Or in this case, a free game in your game demo. It's free, jesus.
If you haven't seen me, DANE, sneak, and some others in TF2 for the past few days, it's because we've been playing the ME3 demo's multiplayer, which is actually just a co-op wave based survival mode, like Gears of War's Horde mode. 10 waves, some randomness, leveling characters, character and ability customization, difficulties. Doesn't matter if you're all about ME's single player, this is fun!
Only problem is that you need Origin, but hey, ME3 won't be on Steam anyway since EA is trying to require all EA games be on Origin, so might as well get used to it (as much as I love Steam). Grab it here:
http://masseffect.bioware.com/about/demo