Here's an intriguing one that sprang up on me while searching the web for newspapers' April Fool hoaxes. It's pretty simple - but that's enough to keep me amused :)

Do a Google search for April Fool, then look down the page for the Wikipedia entry. Everything as it should be. But now try the same search on Bing and look down for Wikipedia. Even stranger, it was doing the same thing on Yahoo until I checked again just now.