1. Someone who works on Cray supercomputers. More specifically, it implies a programmer, probably of the CDC ilk, probably male, and almost certainly wearing a tie (irrespective of gender). Systems types who have a Unix background tend not to be described as crayons.
2. Formerly, anyone who worked for Cray Research; since the buyout by SGI, anyone they inherited from Cray.
3. A computron
(sense 2) that participates only in number-crunching
4. A unit of computational power equal to that of a single Cray-1. There is a standard joke about this usage that derives from an old Crayola crayon promotional gimmick: When you buy 64 crayons you get a free sharpener.