Nitrogen - Has a lot of cheating going on through the site. One issue is multi accounting and the other is collusion.
The cheats are pretty bad though,so just keep a eye out for the min bet in early position to arouse play and the later position over play or reopen for the early position to shove with. Its used a lot and I mark all cheating accounts and prefer to play headsup mostly.
Seals - Is a dead site and is a little risky due to how one of their spokes people got raided,it has never been the same.
Betcoin.ag - Is not bad but again be alert to cheating,it is taken less seriously in the bitcoin world.
Its also not as untraceable as people think,so learn about it before going full hog and getting your account emptied by a hacker.