I'm not going to attempt to catalog every April Fools prank that happens. Other sites have that covered, such as aprilfoolsdayontheweb.com and wikipedia. But I'll list a few of what I think are the better ones:
• The BBC has captured footage of flying penguins. As reported by telegraph.co.uk: "Camera crews discovered a colony of Adélie penguins while filming on King George Island, some 750 miles south of the Falkland Islands."
• YouTube UK has rickrolled its visitors. Clink on any of the featured videos and you'll be sent to the infamous Rick Astley video.
• Google Australia has debuted gDay technology "enabling you to search content on the internet before it is created."
Update: A few more April Fool's Day hoaxes from this year.
Nestle announced they were changing the name of the Butterfinger candy bar to "The Finger," in order to give the candy "a shorter, more contemporary name."
Guinness ran ads in some British newspapers announcing a new drink, Guinness White, available for a limited time only. (As a Guinness drinker, I had to include this one.)
A Russian newspaper, Noviye Izvestia, reported that maternity hospitals had been ordered to play the national anthem every time a baby was born, in order to foster patriotism. Given some of the bizarre laws in Russia, this seemed believable to a lot of people. When reporters queried government officials about it, even they weren't sure if it was true or not.
Hillary Clinton challenged Barack Obama to a "bowl off." She said, "today I am challenging Senator Obama to a bowl-off. A bowling night right here in Pennsylvania. Winner take all. I’ll even spot him two frames. It’s time for his campaign to get out of the gutter and allow all of the pins to be counted. And I’m prepared to play this game all the way to the tenth frame." I think Obama should take her up on the challenge.
Virgin Airlines announced in Australian papers that they were introducing a "no chair fare." Half-price tickets for anyone willing to stand for the duration of the flight. They said that about 1000 people responded to the ad and tried to buy a ticket.