There have, yeah. A few have been legitimate/legal resurrections, where a new company or the same dev team managed to bring the game back somehow. Others have been more grey revivals, emulators and what not in a case of "Keep Circulating the Tapes."
I'll list the white-hat cases I know first.
Meridian 59 was one of the world's first MMORPGs, and it's also probably the earliest example of a game that was shut down and then brought back to life. Meridian 59 dates back to 1996, back to the days of Ultima Online's launch, and was initially developed by Archetype Interactive and published by 3DO. 3DO shut the game down in 2000, but in 2002 a new company called Near Death Studios (founded by several of M59's developers!) bought rights to the game and brought it back.
Near Death Studios died in 2010 (ha), but they left M59's lights on. Two of the game's original technical developers continuted maintenance of the game, and then released it as freeware back in 2012.
This is our grandaddy, ladies and gentlemen.
Myst Online: Uru Live is Cyan World's MMO Adventure Game. Or, more like the MMO version of Uru: Ages Beyond Myst. It's like the Armament Armed Arm of MMOs: it died three times, and was reborn twice. At first, MOUL was supposed to be the online component of Uru: Ages Beyond Myst, but it was delayed, canceled, and a stand-in online product released to customers called Until Uru. Then, in 2006, GameTap reached a deal with Cyan where they would host servers for MOUL. The
only reason GameTap was interested in doing this is because, like us, Myst Online has an extremely dedicated core of players who would not let the city of the D'ni die.
GameTap hosted MOUL for a few years (which had a very GM-driven storyline involving the D'ni Restoration Council and the D'ni's former slaves, the Bahro) until they had to shut it down. For financial reasons, I guess. There was no bad blood between Cyan and GameTap, from what I understand. However, because that same fan base was still doggedly standing by Uru, Cyan came up with a new plan: put the game in the hands of the
playerbase.
In 2010, Cyan announced MOUL's return, having bought the rights back from GameTap. Their goal was to transition MOUL into an "open-source" MMO, in which the players themselves could create new Ages and the like. Their reasons for doing this was, again, because of the determined and dedicated fan base ... but also because right now, Cyan is in a financially bad spot. They can't afford to host MOUL as they would traditionally. Anyway, you can play MOUL legally for absolutely free, as the site says. They just need donations to keep the cavern's lights lit.
The Saga of Ryzom is an MMO I know little about, other than that it was launched in 2004 and has had an incredibly rocky history. It was
almost the first MMO, I believe, to be purchased from its developers by the fans in the Free Ryzom campaign. That happened because by 2006, Nevrax (the developers) were going bankrupt and were about to be bought out. The Free Ryzom campaign had the support of the Electronic Frontier Foundation, but ultimately Gameforge AG bought Nevrax, then spun it off into a subsidiary that
also died not even a year later.
The game was dead for some time, I believe, until Winch Gate Property Ltd. brought it back in 2009. A year later, they worked with the Free Software Foundation to host the game's assets. There's also something they're doing with open source, but I don't know if it's like MOUL.
I'm not sure if
Neocron and Neocron 2: Dome of York ever
died, but they're still kicking and they've outlived their creator, Reakktor GmbH. I'm not sure who had what control to what assets, but according to their current About Us page, the present team has a working agreement with Reakktor's former CEO to continue running and developing it.
Right now, Neocron 2 has been transformed into the Neocron Community Edition, and there's been a push to advertise its resurrection. So maybe the game
did die at one point ... I don't know. All I know is, this sucker's old. Like, 2002 old.
As for grey hat revivals ... Earth & Beyond was mentioned, as was Star Wars Galaxies, but there's also Phantasy Star Online in the form of
Schtack. I generally don't like people who run emulators and private servers
while a game's legit servers are still running, but the guys at Schtack were pretty noble about how they went about it, focusing only on versions of PSO whose servers had been retired by SEGA. That now includes all of PSO, since PSO: Blue Burst's Japanese servers were shut down around 2010-2011ish, I believe (and the international PSO: BB servers were taken down like not even a year after their launch).
There's also
The Matrix Online, which has an emulator in development. I don't know if they've gotten anywhere with it or not, but I may look into it when I have sufficient free time.
I recently heard about
Jumpgate: Resurrection, a sort of space sim MMO that takes after Elite and other games. I can't tell if its current owners legitimately own it or if it's an emulator, but according to the About Us, the game ran from 2001 to 2012, developed by NetDevil and hosted by various companies, with NetDevil working with the GMs of those companies to transfer accounts to new servers in turn. At least until the game finally shut down in 2012.
I'm sure there's a lot of other grey-hat revivals out there, and very likely a few more white-hat ones ... but those are the only ones I can think of at the moment (and I had to look most of those up to refresh my memory). Aside from City of Heroes, I'm keeping my fingers crossed to see Tabula Rasa, Auto Assault, and Exteel come back, personally ... Phantasy Star Universe making a return wouldn't hurt, either.