I did a lesson recently on Subliminal Advertising and showed my students a PowerPoint presentation with some examples of optical illusions. The idea was to demonstrate that you can't always trust your senses. Now Telegraph.co.uk has put together a photo gallery featuring twenty top optical illusions. You can find more by doing a search on Google Images for 'optical illusions'. Here's one of my favourites:
What do you see? A woman strolling towards a bridge? Look closely (maybe move away from the screen) and a human face will appear.