Imagine that your favorite MMO turns off its servers -- there simply isn't enough support to justify keeping the thing running. The MMO-space is littered with the corpses of decent games (and crappy ones) that for whatever reason, their company decided to shut down. More often than not, these dead games leave behind a small core of dedicated fans. I know folks who loved Auto Assault and were sad to see it go. Others are hoping that Aeria picks up Shadowbane (me included), and even 5 years later, I would love to see Earth and Beyond revived.
Normally, your only recourse would be to start a forum post, or a hopeless petition. Good luck with that. My guess is you are stuck finding another game, but what if that weren't the case? Instead of closing up shop and leaving the game to rot, what if the developer/publisher of a dead game made the code open source?
Now making the code of your favorite, dead MMO would not guarantee that you could resume playing. Someone (and more likely a group) would have to have the time and resources to compile the source code and host a server. Performance would probably be pretty bad, support non-existant and updates would be done when they got done... keeping in mind that everyone involved has a day job.
Still, the computing world is filled with very successful open source projects and so I could envision a case where the right project crossed roads with the right dev team and a just-rabid-enough fan-base to resurrect an otherwise dead game.
Until then, Aeria Games... PICK UP SHADOWBANE! (or Earth and Beyond)