Firefall's a free MMO on Steam I've been trying out. It's pretty impressive for a free-to-play MMO.  Feels a lot like Borderlands, but they give you rocket jets n abilities n stuff, and you can change your class at any time and level them up separately. The game doesn't really win any style points from me, and the shooting isn't as solid as I would like it to be (headshots don't do anything), but it's a huge ass game and you can squad up with your buddies pretty easily once you figure out the kinda weird at times interface. For free-to-play, the only stuff you really haaaave to pay for to get is stuff to customize your character (just a couple extra options you can buy) or if you just wanna get some nice weapons or classes faster.

Otherwise though, it's fun, it's pretty to look at, it's a big-ass, cool world you can explore, and it's fun to shoot things, especially with friends.