I've been playing poker online since June of 2006. During that time I've played on various sites.
The bulk of my playing has been on PokerStars. It was the first place I played as a result of their World Blogger Championship of Online Poker.(Watching Spike's gambling competition show King of Vegas was also an influenc in getting started.) PokerStars has become the dominate online poker site, and unlike some of its rivals managed to survive the US crackdown on Americans playing online poker. As noted in recent posts this is the only site I've ever played on using my own money.
I played on the ill fated Full Tilt Poker. I played on it occasionally, and first had an inkling about the goings on in the US when I was unable to connect to the site. However it's possible Full Tilt might have had trouble even without the crackdown. It was already facing a number of lawsuits, including one from one time Team Full Tilt member Clonie Gowen. PokerStars has picked up the assets of Full Tilt, and apparently will reopen it in November of this year. I never managed to win any money off of them.
Probably the most obscure poker site I played on was ScorePoker. This was a site run in conjunction with Canadian sports channel The Score. I did manage to win a golf shirt in one of their freerolls. One feature that I liked about their software was that if you didn't play at all in the first levels of a poker tournament you would be booted out. It eventually ceased to operate, presumably because the network didn't see any value in keeping it going.
For a while I played on the European based MansionPoker. I signed up for their card room because of the TV series Poker Dome Challenge, which Mansion backed. I did win some money at some point, but eventually decided to stop playing there. I think I may have a software disc they sent me stuffed in a drawer someplace.
Then there was UltimateBet, the site that Phil Helmuth and Annie Duke were the spokespeople for. As I remember it I stopped playing there after its software increasingly disagreed with the computer I was using at the time. UB was another poker stite that was having trouble even before the US crackdown, having to reveal in 2008 that it had been heavily comprimised by cheaters able to see the hole cards of other players.
I briefly played on PartyPoker. I didn't stay for very long, as for some reason I found the table software they were using at the time hard on the eyes.