I can't believe no-one here has tried this yet.  Heard about it on a podcast; it's a pixel-graphics, multiplayer RPG set on a space station.

Astonishing depth and almost impenetrable controls without a wiki open at the same time (start here!), but so much potential.  The game has a fully working space station, and every player is assigned a job, from captain, down through department heads, to the various grunts that staff the departments.  You can start as a janitorial assistant, or be an engineer, or a doctor, or a security guard.  You can even be the station's priest or bartender.  Each department's meant to take orders direct from the captain, and then disseminate them down.  Each department has its own radio network, so you can talk to people nearby or broadcast to the whole station or just your team.  

As a janitor you might just end up cleaning the medical bay of blood and wiping the floors.  As an engineer, you can be sent out to recalibrate the solar arrays and make sure they're pointing the right way.  There's a full Sim City-style utility system providing power and air everywhere, and you can have to sneak off and diagnose faults there before making repairs.  Games can have "traitors" who have a specific mission to endanger the station in numerous ways, while everyone else works to keep them under control.



As you'd expect, some servers are just anarchy.  First one I joined I managed to accidentally kill an engineer with a pen, then steal his ID.  That allowed me to open the incinerator amongst other places, into which I placed a clown before venting him into space.

However, I've just played on a proper roleplaying server, where everyone sits in character, knows their jobs, and works at it.  It was amazing; when I accidentally-on-purpose hit a security guard with a crowbar, I was chased, stunned, cuffed, taken to interrogation, allowed my say, stripped of my belongings and thrown in a cell.  Then they even sent a doctor to patch me up and then escorted me out again afterwards.  



It runs on the BYOND system, so you download the BYOND client and search for games through its website.  I thoroughly recommend it; it'd be awesome with a dozen or so forumites.