Pokerslug (http://pokerslug.com) is a heads-up web-based play money poker site. It's organized into "matches", where each match is you vs. a certain player, with a given stack size). Each match can have multiple tables open - usually 10 or 20. Matches are not played live - when you log on, it shows you all the open hands and you play one action in each one.
If the other person is online at the same time, then by the time you get to the end, the other person will have made some moves, too, and you will just keep going. So you can play continuously. When you get into a hand you can see the full hand history so you can make your decision.
It has nice graphs & history features and stuff. I have played about 10-20k hands there in a bunch of matches.
My favorite part of it is EV Poker - this is an optional setting which makes it so that whenever you are all-in, the pot is split based on equity, and no actual cards are dealt. It doesn't change optimal play, and reduces variance. I have played long, fun 10bb games with this, and it is really tough. It's great, though, because there are fewer bad beats ( although they can still happen if both players are not all-in yet), and you can play "closer to the edge" of what you think is the right play.