Do you have a colleague who uses Mac OS X? Next time they step away from their desk, press CTRL-OPT-CMD-8. This will invert the colors on their screen, as if they're looking at a photo negative. To restore the proper colors, just press the four buttons together again.

This and other Mac OS X pranks can be found here: http://www.zaphu.com/2007/08/22/mac-os-x-pranks/