I love this: it's a stick-on USB cable that'll allow you to make pretty much anything look like it's designed to plug into a computer. A banana? Sure! Nail clippers? Why not? Your cat? As long as it puts up with it!
For a mere $8, you can confuse the hell out of your coworkers by "plugging in" whatever you have handy to your computer. Better yet, you can hand them something nonsensical and ask them to pull some data off it. When it doesn't work, tell them it's an issue with their computer. It's certain to drive them insane and waste a good chunk of their time, and isn't that the sign of a great prank?