My first real internet community was an email list I joined in 1993 for discussion of classical Greek and Roman literature, language, history, and archaeology. I was one of the few members who wasn’t an academic.
25 years later, I’m still a member and the group is still thriving. 
There are still plenty of email lists around. I like them and they work well.
The first that I can remember being involved with in a meaningful way was rec.music.tori-amos on Usenet. RMTA had pretty much the group of socially maladjusted weirdoes you would expect from Tori Amos fans, with some really nice conversations about her music and videos, social topics, fan art, as well as a sprinkling of total creepers. We even had t-shirts!
I had my first/only cringeworthy-in-hindsight Internet romance on that board.
Hmm… Not sure I can remember exactly. I tried to get into usenet and irc but never really gelled. Later I found ICQ, but that wasn’t really a community.
I think the first one I actually contributed to and got responses from regularly was the Adobe Photoshop Mailing List in 1997. I think this was before Adobe had any other products at all, and I was using version 3 initially. I remember Illustrator and ImageReady, but they were in the wake of Macromedia Freehand and Fireworks, which they subsequently acquired.
I learned a lot about Photoshop from that community.
The next big one I joined was TheForce.Net, a huge Star Wars fan community. That was particularly busy in the lead up to Phantom Menace in 1999, when fan films were huge deals.
I lurked and then joined the SDMB very soon after that. I think I also got into Hissyfit and Fametracker around then.
The Wisconsin chat room on AOL in 1994. We had some pretty fun IRL parties for a couple of years before things fell apart.
alt.callahans for me. Same nom de net, too.
Don’t remember that. Was it a way of getting on, or for students? I was a grad student then, and I mostly used of of the PLATO terminals in DCL. I took the PLATO class in early 1974, and since George Friedman liked my column he kept my css login active all the time I was at Illinois.
Hah! There was a set of Bell Labs internal usenet groups also. Every year there was a flamewar about whether AT&T should put up Christmas trees in the lobbies. It was so regular that it had a starting date, and people counted down until they could start complaining about this.
First community was VikingMUD in college. I suppose you could consider that a community.
First MB was http://the-park.com, which has been defunct for a long while now.
It was a single terminal in the Union lobby running a dedicated program which provided information about upcoming campus events, movies, restaurants, and other supposedly useful things for students. You didn’t have to login to use it, but you could only access this one program. I only learned recently that it is now considered the world’s first interactive kiosk system, although I didn’t view it as anything particularly groundbreaking at the time. I think the Hotline terminal was set up in 1977 or maybe 1978. I mostly used the terminals in DCL too.
MY first online group was The Book Shelf chat room on AOL about 1995 or 1996
The old h2g2.com. This was back before the BBC buyout, 2000 or 2001. I think I made my first online friend there, and even though we’ve drifted apart it was still a nice time.
ISCA BBS based at University of Iowa, from about 1993-1995. I really don’t remember that much about it anymore, except that I participated reasonably regularly, enough to talk to some members on the phone IRL, and that my user name was Sicarii (or at least that’s what I think it was), but somehow I eventually lost interest. Maybe it was when the Web came into being. But I didn’t really participate in any online communities until this message board after ISCA.
The Buzzsaw boards on AOL back in late 1995 or very early early 1996. I was happily a Sawn until they closed up five or so years later. You’d of liked the Sawn, they were a lot like Dopers (and I know at least another poster or two from there ended up here too).
It probably was the first interactive kiosk. Lots of stuff on PLATO was the first ever, and itsn’t recognized enough.
CompuServe, Prodigy and AOL for me, not counting using a mainframe for work stuff.
Mostly for early games, I remember an ASCII based “combat”. game that literally took all night to play one turn.
It sure was exciting to get that 14400 baud modem!
The Melvyl computerized card catalog question and comment function. It only interlinked the libraries of the University of California, and you couldn’t use your name in a post, but you could leave a nym and conversations could be had through the search function once a nym or conversation topic was known. I think it was used like that for two, maybe three years. Then the need for storage space lead to a purge of all messages that weren’t library questions or answers. Pity. It would be interesting to see how the little micro-community evolved.
I think I stumbled across it when I whimsically typed in Doctor Who as a search topic.
That was probably in the early eighties.
Like many of you, I was active on various BBSs, in the late 80s and early 90s. On the “real” internet, I was very active in… wait for it… alt.pizza.delivery.drivers. I also spent time in rec.music.progressive, though I din’t post there as much. There was also comp.sys.y2k? Or something like that. It was originally supposed to be for discussion of fixing computer infrastructure prior to 2000 (being a comp group and all), but it got taken over by nutjobs who thought the world was going to some to an end on 1/1/2000. It was quite entertaining, though I rarely posted there.
God, some forums in Prodigy. Probably political in nature. Back in the late 80s.
Mine was actually a commercial platform called the Internet Chess Club. It started in 1992, but somewhere in the mid-90s had a massive software upgrade, and that’s about when I joined. It did have some chat room facility, but early on the player base was small enough that you got to know the other players, many of them non-US, and conversations would form either 1-on-1 or in a public form. You could also get into pleasant conversations with International Masters or Grand Masters.
There was for some reason very little trolling, at least until they really started to scale in the early 2000s…but I really enjoyed that place.