OtherSpace Interactive SF Saga
Otherspace is my only real experience with online RPing. There's an interactive fiction MUD I go to (not very often, since I found OS!), but that's chatting and playing a few things online- no roleplaying. There's one or two player-killing code-intense type MUDs I looked at, but they didn't hold my interest.
I originally found OS from a posting to the IF newsgroups; it sounded interesting, so I looked up the website. Once I read a log or two and glanced at the various races, I was hooked. I learned as much as I could about the theme, all of it utterly fascinating. The admins are helpful, player-killing is rare, and it never lacks for role-playing opportunities. Some things you have to give up so you have for time for other things. Otherspace is what you give up those things for.
It's completely original theme. There's a multitude of player races, plus some NPC races. A bunch of planets and ships, and plenty of other players mean the fun never fails. This has addicted me as very little ever has, I generally go on most days for over an hour. As I said, I have little MUSH experience, but it's hard to imagine anything else being as good as this. The admins are dedicated to their job, stirring up RP, coding, handling problems and helping newbies. Did I mention it's very newbie friendly? This is definitely /the/ place to go. If you haven't tried it yet, at least look at their website, www.otherspace.org. You'll be glad you did.