As Poker Orifice has written, at least 4 networks are generally available to US players, possibly with exceptions for individual states. The two major operators GG Poker and PokerStars are not available, but the 4 available networks have a volume of players and game selection comparable to sites like 888 Poker or PartyPoker. So the options for US players are not THAT limited.
What has not been discussed so far however are the limited options for deposits and withdrawals. Since US law specifically prohibits US banks from facilitating transfers to and from
gambling sites, US issued credit cards and wire transfers can not be used to deposit or withdraw on those sites. Or at least so I understand. And this mean, that generally US players will need to deal with either crypto currency or some kind of e-Vallet.
I live in Sweden and opened an account on ACR Poker a few years back. And while they accepted my swedish credit card for a deposit, my only ways to withdraw are those mentioned. So it is a good idea to check the withdrawal options before depositing. And also not deposit more at a time, than what is needed to buy into the games, you want to play. If you want to have a dedicated poker
bankroll, it is best the keep the bulk of it somewhere else than on unregulated offshore sites.