PokerStars. Enough said.  Where everyone plays not just the WSOP Main Event Champions.  If you want a ridiculously busy site then this is it.  Games available all day for every single variation you can imagine.   The Tournament site.

Mansion Poker. Sponsors of Spurs (Tottenham Hotspur FC).  Just moved to the awesome Ongame Network and with promotions and and value special Tournaments you have to play on this site.  They also offer Casino and a SportsBook.

Bodog Poker .  Oh what an adventure Calvin Ayre has had this year.  Bodog has survived and will grow from strength to strength again.  Lovely quick software and a full Brand with Casino, SportsBook, very pretty women etc.  If you have not played here then worth trying out.

PartyPoker.  One of the biggest out there, immense liquidity (player numbers).  If you want to be guaranteed a game or a big Tournament then this is the site for you.  This is the site that most people start on and some never leave.  They also do PartyGammon, PartyBets, PartyBingo and PartyCasino.  The Poker Bonuses are easy to earn and paid out instantly when you have hit them.