Hmmm, well I'll say that I was there for the end of SWG which there was no excuse for. Basically Sony killed the game they had built back up because they allegedly didn't want to renew the license although for whatever reason they kept running "Clone Wars: Adventures". There was a niche for this along with Old Republic Online, but Sony being Sony did the dumbest possible thing, not really giving a crap about it's player base. Even now I think with a graphic update (like they did with the original Everquest) they could bring it right back where it left off and with promotion it would succeed since it can scratch an itch TOR fails at.
That said, as others have pointed out most MMOs get taken down when they have no player base. We're currently not looking at a situation where a virtual world can be abandoned by it's creators, or at least not a professional level one like an MMO due to the way server space works. Perhaps as cloud-type technology increases in sophistication you will see situations where dying MMOs are simply left running indefinatly with their cash shops disabled and the doors open. It's also possible if the laws catch up with the realities of virtual business that ideas like mine will be embraced where games that sell virtual property need to be held responsible for that property existing in perpetuity. Meaning if you say buy a costume in an MMO you need to be able to access that costume any time you decide to play, even if you take a 20 year break. One way to ensure MMOs run perpetually is going to be requiring companies producing them to establish a trust capable of ensuring a server always runs and someone can be hired to maintain it (even if you have one old dude out in a shack drawing minimum wage from the trust in the end). Right now we haven't seen any incidents big enough to get governmental attention yet though. The subscription based model of some things like SWG made them relatively easy to turn off, and most other games that died were a relative drop in the bucket if they had virtual property. When something like "Star Trek Online" dies for one reason or another, it might be different though, because while only a "good" game as opposed to a great one, it has that Star Trek license and as a result some truly fanatic players, many of whom have literally put thousands of dollars into the game. If Perfect World (the guys calling the shots more than Cryptic) ever decide to let that one die now with the current business model, I suspect it won't go as quietly, there won't be riots in the streets or anything, but I suspect a lot of people will say "hey, I know this is legal, but it really shouldn't be and here is why, and what I paid..." a few interested politicians and views of all the money invested across the MMO spectrum and I can see laws existing requiring businesses to protect virtual property, especially with the business getting bigger and bigger, and how it's inevitably going to move beyond just recreational games.
That said I'm rambling and getting away from what I originally intended to mention before I got off on my tangents. The place to look for abandoned virtual worlds is probably MUDs, not MMOs. They might be hard to access nowadays (I haven't tried for a long time) but MUDs were basically the way people did multiplayer computer RPGs online before MMOs and graphics. Indeed the skills and such in Everquest (Kick, Bash, etc...) and terms like Mob all come from MUD terminology. MUDs were entirely text based, like old Zork games, but could involve incredibly sophisticated mechanics especially seeing as they didn't have to worry about things like visually showing what they were doing. A high population MUD might have 50 or 60 people on at once (some could boast hundreds though) and a few were quite huge. That said even when I did MUDs there were a lot of abandoned ones, and of course when looking part of the trick was to find a good one that had a lot of players, but not so many than you couldn't reliably login. Given that many MUDs were located on university mainframes and continued to grow over generations of students when they were popular, and then die out entirely when they weren't, it wouldn't surprise me if you could find MUDs now with millions of play hours on them that are entirely abandoned, and wander through them realizing that someone actually sat down and had to type every bit of description (some being quite picky on writing quality), program every mobile, create every item, etc... and in some cases even simple things could take a lot of effort. Good MUDs had both builders creating things, and Coders making code to change how the game worked and develop various subsystems.
Not as flashy as MMOs, but if you want to walk through some dusty ruins, that's the place to go. Sometimes I've thought about doing it, but honestly the last thing I need is more depression in my life, so even if I could find them again, odds are the success wouldn't wind up making me happy.