The only great April Fool's joke I was ever a part of took place in 2003. I was interning at the International Tribunal for the Law of the Sea in Hamburg, Germany. A fellow Dutch intern and I decided that the place needed something to spice it up.

It was the start of the Iraq war and of course I don't have to tell you what everybody's attitude towards that was seeing as it was an international body under the U.N. system. We would have daily lunches sitting around watching the war progress on the BBC or CNN and catch snippets of our favorite Minister of Information, Comical Ali Al-Sahaf. Among the constant influx of war rumors there were some rumors that Iraq had put the WMD's on a Yemen ship in order to hide them from the inspectors and later the invading infidels. So my partner and I decided it was time to bring the war to the Tribunal which was so pathatic in its grand building and 21 international judges sitting around waiting for a case to be brought in front of it's panel.
Here's what we did. We made a web page that resembled CNN's front page and made the headline read "WMD's Found in Indian Ocean." The big headline story had a big tanker for a picture and linked to the story which contained the kicker. The story went on to say that the US had seized a Yemeni tanker in the Indian Ocean, boarded the ship, discovered Iraqi WMD's and had US officials making preliminary statements about finding the "smoking gun". We then inserted ol'Comical Ali into the picture. Yada yada yada..."We will take this breach of our sovereignty on the high seas by the infidels to the International Tribunal for the Law of the Sea!"
The best part was we were able to get into the Press Officer's email account and send it to all of the legal officers and Registrar (who is basically the head of the whole Tribunal after the President), giving it authenticity. We carefully monitored the legal staff and we waited for someone to find the story. When they did it was fantastic. The whole legal department, Registar and President were gathered around a computer gleefully discussing the story and that the Tribunal might be getting a real case! These people were world class scholars and they were acting like children on Xmas morning. My partner in crime watched with wringing hands. We walked in and when they asked if we had seen the story we just couldn't keep it in and well, "April Fool's!"

After all the confusion, as many of them didn't know what April Fool's was, The Dutch Kid and I were showered with praise for our little gig.
Good times.