Johannes Hjorth of Sweden posts a great guide to growing cress in a co-worker's keyboard:
"My colleague Erik Fransén went away on the Society for Neuroscience conference in the USA for two weeks in the middle of November. Those of you who know Erik probably know that he enjoys playing practical jokes on his friends from time to time. This time it was Erik's turn to be on the receiving end..."
Classic. (Hjorth posted back in '05, but I think we can all agree, it's still gold.) Many more office pranks to be explored here.