I'm surprised nobody has posted about this, so let me do the honours
Do you guys remember City of Heroes: Icon? You know, that little hack which allowed you to get into the City of Heroes costume creator even though the servers for the game no longer worked? Well, the folks behind it have expanded its capability. At this point, you can get into the actual city zones with the character you created and walk around. Crucially, the game is now hooked up to an XMPP server which allows you to chat with and even interact with other people from inside the game via global chat channels.
The game itself doesn't work, obviously - none of the server backend infrastructure is there to handle... Really anything. No powers, no enemies, no combat, no inventory, not even any NPC interactions such as with trainers and the like. This is strictly a 3D chat room with custom-created avatars which exists primarily for the sake of nostalgia. If anyone is interested in checking it out and maybe taking a stroll around Atlas Park once again, here are the official instructions.
Paragon Chat is an offshoot of the broader attempt to duplicate City of Heroes' back-end server structure via open-source, player-created software and one day run the game that way. That seems to be a ways off, but it's still being worked on, clearly. If you want a shot of nostalgia, give Paragon Chat a try. I personally walked around for a bit, but sadly - prime time for that is middle-of-the-night for me
(updated site tags by Pax, no other edits made )