According to Poker Scout, it looks like the Chico Poker Network (with the skins Bet Online, Sports Betting, & Tiger Gaming) is firmly the second most popular site where US residents can play nowadays. It's behind Bodog where players are anonymous and thus there are no huds ... which, in theory, makes Chico the most popular US-friendly site where HM2 could work. It looks like it's been gradually gaining traction while Winning and Merge (where I play) have declined. Sigh.
Any chance HM2 will support it at some point? I can't speak to the quality of the site or the software or anything else. Just looking at the traffic stats and am a little intrigued.
(Man oh man, it sure does suck being a US resident trying to play poker these days.)