It's free, 2D, and no one is an NPC.

http://www.byond.com/games/hub/Exadv1/SpaceStation13
(and on how to play it): http://www.declassified-ss13.com/index.php

The Persh server (which gets most of its users from the SA forums) is easily the best.

Jim Jones Muhammad- [145.9]-broadcasts: Hello station, I am your doctor and hate death. So do not die.
Roland T. Dapperman- [145.9]-broadcasts: What do you think about comas from which there is no hope of waking up from?
Jim Jones Muhammad- [145.9]-broadcasts: In the event of that happening, that'd be like some unstoppable force of annoyingness due to a disputed 'death.' Ergo we shall throw the coma'd body into space.
Harry Block- [145.9]-broadcasts: or eat it
Jim Jones Muhammad- [145.9]-broadcasts: And create a master race.
Harry Block- [145.9]-broadcasts: nice

That evidently doesn't capture the awesomeness of the game, but one thing is that this game is by far the most grief-friendly game ever conceived. For example, the station can actually be completely destroyed, or you can break a wall down. You can go into engines and burn all the fuel (or just destroy the engines) thus making electricity not work, which shuts down most types of doors, obviously all the lights, computers, you can siphon the air out of oxygen tanks so when people use them they'll suffocate pretty quickly, you can start fires, you can make bombs, you can wall off areas, you can put on a gasmask and use someone else's ID to disguise yourself etc. Not to mention the sheer amount of stuff you can do to other players.

A random example from a few months back:

When game began there was no Captain, so I declared that Syndicalism would become the official policy of SS13. (Even though I was just a station technician) Within 5 minutes I had 6 people in what I called the Assistant Labor Federation. however, security began to get scared (I called for them to disband) so they tried to arrest us. After killing one, the rest of security (and the only person with higher authority, the Head of Personnel) called for us to disband. The military (who had no idea where SS13 was and thus never came to put a stop to the ALF) even threatened to blow up SS13 if we did not disband.

I went into bridge (Captain's area) and called on the two security guards and HoP to surrender their IDs. By then three ALF members (including me) had tasers. Said security asked me to leave. I refused, they tased me, arrested me, and the ALF uprising began. Eventually I was freed by ALF members and uncuffed, as was another ALF member.

Eventually I called on the Federation to storm the bridge and by then we had 8 members. I walled in the HoP (who was calling for backup in vain) and declared victory. Everyone who wasn't in the Federation (and was either dead or intimidated to the point of hiding) demanded a reboot and it was so. Only three people died: two security, and one sadly normal station assistant who sided with security.

Another random game is when I named myself Michael Jackson and would tunnel into the research lab and steal monkeys (the only NPCs in the whole game, and even then it's possible for players to be them) and hide them, which resulted in me doing everything I could to hide the monkeys (and myself) from security. Eventually I fled into toxins lab, made a bomb, and threatened to blow myself and all the monkeys up. After about a minute of negotiations, security announces they're coming in. I detonate the bomb, causing a fire as nearby plasma tanks burn. After security breaks open the door, the fire bursts out of toxins and kills all 7 people by the door, including the entire chain of command.