Rotten Potatoes replaces existing links on web pages with a Rickroll. You can customize the redirect URL.
Looking for a way to Rickroll your friends on April 1st? Look no further than the Rotten Potatoes extension! This extension replaces all links on every website with a Rickroll or your custom URL on April Fools' Day with the press of a button. Open-source code available http://rottenpotatoes.me.