The only free to play games I've played are Team Fortress 2 (but I bought it, so I don't think that counts) and League of Legends. I also tried Age of Empires Online, but it felt like they were taunting me into paying them (quest rewards I can't use, and the like). I wanted to ask the rest of you:

What free to play games are worth playing? MMOs, shooters, arenas, etc.