PokerNite

Simply walk up to a seat at the table and choose Join Seat. You will replace the AI player already there. (You can't replace a real human player.)
If you need a refresher on how to play Texas Hold'em, check out the Training Room! There are help screens to give you a leg up on the competition.
Hand Analysis and PreFlop Charts now available in the Training Room! AI has also been updated to be tiny bit smarter and less annoying.