Wikipedia - Poker
Information about Poker, a card games involving betting and individualistic play. The most popular form of poker is Texas Hold'em.

Wiki Poker Encyclopedia
Inspired by wikipedia offers up to date source of information about Poker in addition to the ability to edit and add poker events, poker players and more.

USPC - Poker Games
This page is from the United States Playing Card Company, and contains a thorough discussion of the game of Poker, background of the evolution of the game, and rules.