Depends on what you like.
If you're more the FPS type I would recommend Crossfire. Then there's always Team Fortress 2 which I would personally recommended as well after putting hundreds of hours into it.
For a TPS I'd go with S4 League. Could be considered pay-to-win, but it isn't too extreme. A lot of hackers on this one though, especially on the Rookie channels. Good game all around though, especially if you can get into a good match.
If you drift more toward fantasy/RPG type games I'd go with Mabinogi. It could be considered a pay-to-win game depending on how you look at it though. It's taking up a lot of my time previously, it's a good change of pace compared to other free RPGs.
If you want a RPG type game with more of an action feel to the combat go with Vindictus. I haven't had much experience with it since Beta but it was extremely promising and still looks like good fun even now.
Then, if you were interested in trying it, there's World of Warcraft. Which is free to play up to level 20 I believe; its also one of my personal favorites. I still subscribe to it now.
Can't go without mentioning Warhammer Online. Mileage may vary on this one, the endless trial offers a lot for only allowing play in the first tier; personally haven't tried the other F2P game they have out though.
On the non-MMO side, I can't really name a lot. There's a free Trackmania game on steam I believe, which is racing if you haven't heard of it. There's always the free flash games you can find here on Newgrounds, and this little gem called SCP Containment Breach, your mileage may vary on this one.
And that's all popping out at me for the moment.