partypoker was the largest online card room until 2006 when it left the US market due to the Unlawful Internet Gambling Enforcement Act of 2006. PokerStars has since claimed the title.

Make sure you're legally allowed to play\n\n Party Poker is only legal in New Jersey and Pennsylvania at the moment, although it plans to expand to other states soon. You are allowed to register for an account if you live outside these states, but you are not legally allowed to play unless you are within the state borders.