If you've played Knights of the Old Republic, then you'll have no problem remembering HK-47, the talkative, if psychopathic robot who called you a "meatbag" and threatened to kill everyone all the time.
He was also a part of a scrapped portion of the game's sequel, which had him cloned into an army of HK-47s hellbent on taking over the universe or something.
The Hunter Killer robot will be making a return of sorts in BioWare's MMO, Star Wars: The Old Republic, which takes place some years after the events of the two games.
This time around, you'll have the chance to recruit HK-51, which has presumably been cloned en masse to provide scores of players with the opportunity to recruit him. You'll have to do a bunch of quests (details of which have yet to be revealed) in order to unlock him and have him accompany you.
He'll be available shortly in an update for the game.