When online poker first came into existence, and this is before the MoneyMaker boom, every room in the world graciously accepted players from the United States. Advertisements for the online poker rooms were on television, and online poker rooms sponsored Major Land-based poker tournaments. The Party Poker Million, is an early example.
Then, the United States government took the position that online poker is gambling, and not a contest of skill between opponents. And, that online poker rooms who accepted play from U.S. Citizens were potentially subject to legal action. That threat was enough to convince publicly traded companies to remove themselves from the United States market.
Party Poker stopped allowing U.S. Citizens to participate in real money gameplay. Other rooms followed, such as 888Poker and Titan Poker.
Still, several key rooms in the world of online poker continued to accept players from the United States. These included Poker Stars and Full Tilt Poker, both of which grew to be the largest online poker rooms on the planet.
Recently, the U.S. government seized the website domains of several major online poker rooms, and brought charges against Poker Stars, Full Tilt Poker, Absolute Poker, Ultimate Bet, Doyle's Room and True Poker.
Both PokerStars and Full Tilt Poker have stopped allowing players from the United States.
At this point in time, there ARE other rooms who are accepting players from the United States. But, I cannot recommend that anyone Deposit and Play at these rooms.