Depends on where the story I'm writing is set. There are lots of great suggestions for historical and modern names already listed here. In my own world I just make them up by looking around the office. Right now I'm looking at a doorhandle. So, jig the letters about a bit and, presto, the name I...
The Assassins (as mentioned in Johnny's link also feature prominently in the history of the Crusades. Their leader was called The Old Man of the Mountain. Now how's that for a title.