There's a platform on the internet that allows you to create rather complex multiplayer games, even persistent ones, called BYOND - in fact, some time ago, we even considered it for making a new Seed. Well, someone almost beat us to it - have a look at this: http://www.byond.com/games/Exadv1/SpaceStation13
Okay, that, and the wiki linked, won't tell much, so let me do it instead. It's a game where you play as a crewman on a space station where something bad is about to go down - traitors, AI going rogue, some weird biohazard outbreak, random psychopaths deciding to make some bombs and whatnot - fun (and probably gruesome death) for everyone! The rules and mechanics involved are quite complex, and it is sometimes described as "Dwarf Fortress in space!" - of course it's an exaggeration, but it's not that far from truth.
It's also supposed to be RP-enforced, although there isn't much background material, and exactly how "enforced" it is depends largely on players and moderators involved.

The game itself is being developed and maintained by a certain internet community no-one particularly likes, so there are some weird features available in the latest build (Like surgically removing people's butts and wearing them on your head - which aptly describes one of the encouraged behaviours) and things usually happening for no reason.
However! It's not very hard to get your hands on a slightly earlier build which naturally lacks some features and start your own server - some other communities have done just that. Including Bay 12, whose server requires you to ask to be let in on their forums nicely enough and reportedly contains less random stuff happening and more serious business RP.

It's fun to play, whether you're doing anything useful or wreaking havoc (In fact, why not do both?), but the learning curve is huge, the interface and controls are extremely unfriendly and bulky, and the servers I played on so far are extremely laggy. Still, fun! Nothing like making your transport robots run over people if they stand in their way and announce over the PA system that everything is working perfectly well and it's their own damn fault. Or mopping the floor in front of the escape shuttle so people slip, fall and don't get on it in time. Or slightly oversaturating the air with oxygen. Or... you get the idea.