I think Ultima Online was one of the first MMORPGs that I played. I think Ultima and Ragnarok Online. I loved the entire feel of Ultima Online, it had a great community. Same with Ragnarok Online (I still help code for my friend's private server, actually). I fell off playing UO once I started getting more into RO, but to be honest, UO and RO are the only two MMOs that have been able to captivate me for long periods of time, never really got the appeal of WoW. This makes me want to find some shards to play some Ultima Online this weekend.
Not the above, but I can chime in. Ragnarok Online private servers are (now) an entirely different beast than any of the official servers.
Automatic events, large custom maps, all sorts of fun player oriented stuff; custom classes, custom gear, new sprites for players and monsters, new colors all over the place, different balancing... All built on top of this ancient engine that's been ported and rebuilt a dozen or so times now.
Reading a bit of Ragnarok Online's private server history is very interesting.
