Matthew Moore wrote a really neat article for the Telegraph on 20 of the most viral Photoshopped photos on the Net. Like this famous photo of a shark lunging at a helicopter:
This striking fake was created by merging two separate images - a US Air Force helicopter on a training exercise in San Francisco, and a great white shark leaping out of the water off the cost of South Africa.
The hoax emerged in 2001, and was later circulated via email with a caption claiming it showed a shark attacking British Navy crew in South Africa, despite the fact that the Golden Gate Bridge is visible in the background.
See if you remember 'em all: Link - Thanks Andrew!