Titan - one of the iPoker network room. iPoker have a lot of online platforms like: William Hill, Sun poker, Chili, paddy powerpoker, bet 365, Noble, Tony G and more.
Full Tilt - is licensed by the Kahnawake Gaming Commission. They have a huge deposit 100% bonus up to $600. FullTilt is independent and one of the large.
PokerStars - is largest platform in whole internet perchaps. There playing about 180k people online now !! This is a home of many World Champions since 2003.
Cake Network - probably smallest network , but have many awesome and trusted rooms : Cake, Doyles Room, RedStarpoker, Sports inter action, sportsbook, Bet US, more.