The Society for Interactive Killing. A spoof (namewise) of the Society for Interactive Literature. SIK games are a class of games run under the MIT Assassins' Guild, emphasizing combat and violence. Where a "normal" game might emphasize roleplaying interactions, gaining political advantage, or stealing technology, SIK games are oriented around accomplishing a mission while still surviving.

Common SIK game mechanics:

Hit Points
Players have some. When they reach zero, fall down. Die in the near future.
Guns
You have some. When the projectile hits someone, they take some damage.
Stim
Use this to give someone some more hit points.
Doors and Wards
Things in your way you have to break through.

Some well-known SIK GM teams:

Demolition, Inc. (aka DemInc)
Purveyors of Antarctic Base, and the Badlands series.
Carl
Plot-heavy SIK games: Caverns and Caverns 2
The Andy Conspiracy
Themed SIK games: Z'ha'dum, Z'ha'dum 2, and Buffy: Armageddon
SIK Bastards: A rotational GM team (one GM writes, another helps run, and the third gets to be a player)
Exodus, things from the Nexus universe, X-Com, et al.