Firefall is still in beta but keep it on your radar. I've had a lot of fun with it.
Team Fortress 2 is the best F2P game ever made.
I've heard great things about Spiral Knights. I had a bit of fun with Global Agenda and APB.
Tera just went F2P and has a great combat system, from what I hear.
Star Wars: The Old Republic is enjoyed by a lot of people. It's certainly one of the prettiest F2P games. I thought it was too close to WOW, which I already didn't like. If I had wanted to play WOW, I'd have played WOW!
Load up your Steam client and hit Games > Free to Play and do some browsing.
Really though, Team Fortess 2, yo. I bought it on 360 and spent 140 hours on it. I bought it again on PC and spent another 50 hours on it before Steam started tracking play time, then another 80 hours since. When it went F2P it didn't change the game, it just brought in more players and very optional
alternative weapons. It's F2P done perfectly. It's a retail game with purchases that do not give anyone an advantage over anyone else. It also does not do any of the standard time-punishment garbage that infects a lot of F2P titles. Also, once you make you first purchase you become a paid player forever. If you spend more than a couple hours in it, spend a $1 and turn your account in to a Premium version.