No april fools, but just as fun: when it's time to set the clocks an hour back, I go around the office (since i'm in first) and set them all an hour forward.

watch the guy that has the job of setting them forward wonder why the clock are still wrong after he went around the office

My best ever April Fool's joke: I meowed like a cat into a tape recorder (meow), once every 30-60 seconds, for about a half an hour. I pushed "play" and turned it up so you could hear it and put it in my briefcase, then went around to co-worker's offices to chat and casually left it along side of their desk. Two of them got a big laugh, but the third thought there was a cat stuck in the wall and called maintenance. She was pissed when they found it, and she rarely spoke to me for years afterward. Worth it!

#14 You also can turn the desktop upside down and invert the mouse movement.