I'm a computer user...nerd. So I know a few useful office work pranks.

•Switch the "m" and "n" keys on someone's keyboard.
•On a mac hit "apple+option+contol+8" to invert the victim's screen
•Unplug the keyboard
•Put clear tape over the mouse sensor.
•Take a screen shot of the victim's desktop and use it as their wallpaper

Non office pranks:
•Fake craigslist ads- I put a car ad up for a Pinto and a Gremlin
•Fake missed connection ad if the victim is known to peruse this section

Of course the prank depends on the sense of humor/tempermant of the victim. Never do something that will make them freak out- or will make them reset their computer losing unsaved work...

Enjoy!

Google is also pretty good today