Dang, you people don't give f2p MMO's a chance. I personally don't like Silkroad Online. But it's up there in the top 10 games most played. It beats Guild Wars in hours played. Another great f2p MMO is Cabal Online. The structure of the game revolves areound PK-ing. It's awesome unless you're a cry baby when it comes to people picking on you over via online game. There's some annoying grinding but over all a great game. I'm listing all the f2p MMO's I can think of in order of hours played.
Silkroad Online
Cabal Online
Maple Story
Runes of Magic
Metin 2
Flyff
Shaiya
Perfect World
Atlantica Online
Archlord
Rapplez Epic IV
Last Chaos
Twelve Sky
RF Online
Twelve Sky 2
Mentionable ones - SRO, Cabal, Runes of Magic, Shaiya, Archlord, and Last Chaos