The Ship mod for Half-Life

Imagine a game in which you don't know who to trust, you don't know who may be about to kill you, in fact you don't even know who is human and who is artificial. Played online, your quarry is another human player but can you find them amongst the AI inhabitants of the Ship? Once you know who they are, find yourself a weapon but keep an eye out for security guards and cameras. If you're spotted trying to murder someone you'll be transported to the Brig and suffer a hefty fine. If you succeed in killing your quarry Mr. X will reward you with a cash prize, depending on how entertaining he found your murder. At the same time you must look after yourself, wash, sleep, eat and have fun and remember - always look over your shoulder because somewhere there's a player looking for you!

This classic mod is now a standalone game built on the source engine.