One of the largest defense contractors that the United States employes, Blackwater consists of former Special Operations troops, law enforcement officers and front-line soldiers.  They protect convoys and military installations and were previously contracted to defend the United States Embassy in Iraq, the largest embassy the US currently has.  They have been specifically linked in the Nissor Square shootings which resulted in 14 civillian deaths.  They have since renamed the company to Xe Services LLC.

See Crocodyl's profile of Blackwater