One of the several "whizkid" staff that I am fortunate to have reporting to me, Matt Batchelder, pulled a prank while several of us were off at a conference. He did want to come to the conference, and should have, but budget realities being what they are he had to stay behind. After hours, he and several other staff had some fun "decorating" the offices of those who went away. He's a blogger and apparently the story has taken on a life of it's own and has made it all the way across the pond and was covered by the Daily Mirror:
Those are some pretty amazing pranks! Another great one is to scotch tape your colleagues mouse, it will still work but not very well. Guaranteed to drive them insane!
This is how to do it: http://www.officepranksmasters.com/Tape_mouse.html