I started on site that I am pretty sure no longer exists, it was a place that linked people wanting to RP through email. I found quite a few decent groups through there, including my first taste of adult RP that wasn't supposed to be quite so... adult, lol. Me and this one other writer hit it off, then our characters, and we were basically cybering right in the middle of some plot I still can't remember, lol. Not my finest moment, but it was pretty fun anyway.
I didn't get really seriously into it until I joined a LotR fan RP, that was a helluva lot of fun and I met some great people. We met on YIM all the time, sometimes to discuss RPs, and sometimes just to chat. I remember I had to break their habit of calling me Frodo because that was the first character I played with them.
That after that, I played in a several really awesome modern fantasy groups, with two characters I grew to like very much, and many others I just played for a time or two. But the groups, all run in the same circle of writers, pretty much imploded after a fair bit of drama, and I decided to leave groups behind for a while and focus on 1x1 stuffs. I met all sorts of wonderful and talented writers, including one who opened my eyes to the possibilities of non-straight sexuality.
Whether group or 1x1, I almost always played free form, with one very awesome and notable exception where I played in a GURPS based game, with a phenomenal group that I got so close to, I went out of state to visit them for a week. We sort of drifted, then I found another group, started a few awesome games, tried to run one myself that didn't work out so well, then moved in with one of them for a while, heh. Since then I have been drifting from site to site, until I finally found E, where I have finally found what I think I have been looking for, for quite some time.
I hope the answers to your questions are in there somewhere within my rambling?