Full Tilt Poker
Offers the best in online poker: world famous pros, a huge bonus, real or play money.
http://www.fulltiltpoker.com/

Poker Stars
The world's largest online poker room, with more poker games at every level, bigger tournaments and more players than any other site.
http://www.pokerstars.com/

Sky Poker
Play online poker with Sky Poker and claim up to $500 free bonus on your first deposit - if you are a novice player or new to poker you can learn in the free poker rooms or by reading the online information and help section. Sky Poker is one of the UKs leading poker provider and people choose them for a specific reason!
http://www.skypoker.com