I've played all the ones on the Top10 list here, and tons others too numerous to mention. The ones I currently actively play are AVA, GenAD, a little bit of S4 (mostly not playing it though) a little bit a MicroVolts just to try it out, I'm also enjoying the CB of Metal Assault which is a lot of fun.
I play a lot of F2P games but those are the main shooters in that genre since we're talking strictly free. I played a lot of Battlefield Heros at first but kind of lapsed out of it. Quake Live probably deserves a mention in there somewhere too.