Some guy posted a collection of optical illusions and party tricks on YouTube. The videos have too many pop-up messages and are best viewed with the sound down. But there are enough videos to kill off 10 or 15 minutes of your work day. The videos average less than 2 minutes each. Here’s one that isn’t an illusion but I’d like to see someone try it out if they happen
More Optical Illusions & Visual Phenomena to mess with your mind. This site uses Flash and Quicktime for a lot of their illusions.