Washington State is the only state you can't.
Now, not ALL sites are US friendly, if the site is in the US, it will be in a regulated state.
You can play at Ignition, American's Card Room, BetOnline and
intertops are the ones I know, now, ACR is really a skin for the Winning Poker Network - and there also exists BlackChip. And yes, you can have an account on ACR, and on Black Chip - so long as you are not in the same WPN game on both, you ARE allowed to play twice. Including
freerolls. Note Intertops has a clone too-
juicy stakes. That one is a bit picky about playing twice in third party sponsored FR's.