I used to play online poker at PokerStars.Net. Then came the Great Preet Bharara Thug-Fest of 2011 because omigodomigodomigod some Americans had been playing with/for real money at the dot-com version of PokerStars without Preet Bharara's permission, so of course he had to seize all of Pokerstars' stuff for the consumerrrrrrs (if you think you've noticed a recurring anti-Preet-Bharara or anti-Catherine-Hanaway thread in my stuff, you're right -- they're gangland terror lords of the first water and when the revolution comes I won't feel a bit bad if they happen to get put up against the wall).
So anyway, once it turned out that Bharara didn't have a legal leg to stand on even in the current "the government can do anything it wants" environment, PokerStars got their stuff back. But by then I was moving off of Mac (for which there was a downloadable Pokerstars client) and onto ChromeOS (for which, so far as I know, there was and is not).
A few months ago I discovered Replay Poker. It's a nicely set up site, has frequent tournaments and "sit and go" games of all the popular poker variants (I'm an Omaha Hi-Lo aficionado myself) and it's "free" (you can buy extra chips if you like, but they're free in limited quantities and of course they are "for entertainment only" -- you don't win money there, at least that I know of).
Yes, it's an affiliate link -- I get free chips ("for entertainment only") if I send them new players. So if you're into online poker and haven't checked them out, consider yourself invited to do so.