Massively multiplayer superhero game City of Heroes celebrated its 20th anniversary this year. It's definitely showing its age -- it's a slightly archaic, chaotic game in which players create their own Silver Age-style comic book heroes and villains and run, fly, surf, and warp around the basic metropolis of Paragon City. But you can still log in and play it, and be part of its small but passionate community.

In itself, this is nothing unusual. World of Warcraft also celebrates its 20th anniversary this year; EverQuest, the game that inspired WoW, has been in continuous operation for 25 years; Ultima Online has changed hands a few times, but has been running since 1997. Provided enough players become deeply invested in these online worlds, they're predisposed to last. Even if the original developer or publisher loses interest, someone else can often be found to take on the server costs and collect the subscription fees.