Bovada/ignition is pretty much the only US facing site worth playing as far as I know. The software is bad and the rake is huge, but at least decent games run. They're probably the biggest network in the US.
Americas cardroom (probably 2nd biggest) has better software and rakeback, but the games are bad and there's a lot of screwy stuff going on there. They lost their sponsorship on the 2+2 forums because they literally just didn't respond to Mason Malmuth when he asked them about some player concerns. (Bots, tournament cheating, and even super users came up, though I don't know of any proof of that last claim.) That's basically the equivalent of if an accredited casino just completely ignoring personal messages from Bryan on casinomeister - maybe even worse because winning poker network had a sponsored forum there.
There might be some other smaller sites to consider, but honestly I probably wouldn't deal with any of them, most of them are either bad or have so few players I would just give them a pass, or both.
Some people play on mobile phone based private games, but unless you're involved in the live poker scene (which I am not) you probably won't have connections to find good games, and they have some of their own problems anyway.
You could check out the 2+2 forums for more on this stuff, they've pretty much got a whole long ongoing thread on where US players can play, but the simplest and probably best answer to your question is "just play on Bovada.
2+2 is probably going to be the best forum resource for most poker stuff, though I think there are a lot more annoying/trollish posters you have to wade through in some parts of the forum these days.
Edit: Here's the 2+2 thread, I'd say skim the last few pages or so if you want to see what people are saying, and maybe look at if any of the threads in the internet poker section look interesting. Or just play bovada:
Edit2: 2+2 statement on ACR:
