1. Prank your friends with this PhantomKey-strokerr V2.
This little device plugs in to your unsuspecting friends computer and randomly moves the mouse, types gibberish, and turns on and off the caps-lock. (it never presses enter or clicks the mouse so at least you won’t cause people to publish or save their now messed up files. You can purchase this at ThinkGeek.com
2. USB Laser Guided Missile
That is actually not a typo. Plug it into your computer, aim using the laser guidance and squeeze the trigger (right click). Enough said. Click here for the link.
3. Free prank
Screen capture an image of the offending coworkers desktop with all their icons. Save it and make that image the new desktop wallpaper. Remove the icons. Now sit back and watch your coworker click icon after icon while frustration grows.