For April Fool's Day 2006 I pulled three "pranks". Ish.
The most traditional prank was "revealing" that my regular Piracy Calculator page into what a large number of people had suspected it to be all along - a criminal-nabbing tool for the RIAA. Note that "Hugh E. Samuels" is an anagram of my real name and that the initials of "Associate Finance Director" are also the initials of "April Fool's Day".
Less a prank than a jape was the International Earth-Destruction Advisory Board finally announcing that the Earth had indeed been destroyed, and changing the EDA Level to "Red". I emailed the mailing list to tell them that it had happened, as well as laying a small amount of groundwork for the page to return to normal the following day.
I always intended to try to tie this in with the main How to destroy the Earth page by using that page to detail how, exactly, the Earth had been destroyed. I tried to write it as a tongue-in-cheek factual writeup but when I realised it was just going to be a boring re-hash of a method everyone had probably read about already, I knew I'd have to come up with a new, fictional method of my own. So I went the whole hog and turned it into a story which is now in my fiction section. At the time I only had the first part done; I had the rest of the story roughly figured out, but I was pushed for time and couldn't post it before AFD.