From museumofhoaxes.com, here's a link to their Top 100 April Fool's Day Hoaxes. Some of my favorites ...
#8: The Left-Handed Whopper
1998: Burger King published a full page advertisement in USA Today announcing the introduction of a new item to their menu: a "Left-Handed Whopper" specially designed for the 32 million left-handed Americans. According to the advertisement, the new whopper included the same ingredients as the original Whopper (lettuce, tomato, hamburger patty, etc.), but all the condiments were rotated 180 degrees for the benefit of their left-handed customers. The following day Burger King issued a follow-up release revealing that although the Left-Handed Whopper was a hoax, thousands of customers had gone into restaurants to request the new sandwich. Simultaneously, according to the press release, "many others requested their own 'right handed' version."
#12: Flying Penguins
2008: The BBC announced that camera crews filming near the Antarctic for its natural history series Miracles of Evolution had captured footage of Adélie penguins taking to the air. It even offered a video clip of these flying penguins, which became one of the most viewed videos on the internet. Presenter Terry Jones explained that, instead of huddling together to endure the Antarctic winter, these penguins took to the air and flew thousands of miles to the rainforests of South America where they "spend the winter basking in the tropical sun." A follow-up video explained how the BBC created the special effects of the flying penguins.
#46: Guinness Mean Time
1998: Guinness issued a press release announcing that it had reached an agreement with the Old Royal Observatory in Greenwich, England to be the official beer sponsor of the Observatory's millennium celebration. According to this agreement, Greenwich Mean Time would be renamed Guinness Mean Time until the end of 1999. In addition, where the Observatory traditionally counted seconds in "pips," it would now count them in "pint drips." The Financial Times, not realizing that the release was a joke, declared that Guinness was setting a "brash tone for the millennium." When the Financial Times learned that it had fallen for a joke, it printed a curt retraction, stating that the news it had disclosed "was apparently intended as part of an April 1 spoof."
#65: Y2K Solved
In 1999 the Singapore Straits Times reported that a 17-year-old high school student had one-upped all the major software corporations of the world by creating a small computer program that would easily solve the Y2K bug. The camera-shy C student had supposedly devised the program in twenty-nine minutes while solving an algebra problem for his homework. His family and a technology consulting group were reportedly forming a joint venture named 'Polo Flair' in order to commercialize the discovery. They anticipated achieving revenues of $50 million by the end of the year. Numerous journalists and computer specialists contacted the Straits Times, seeking more information about the boy genius and his Y2K cure. One journalist even wanted to know if the boy would be willing to appear on TV, despite the fact that he was camera shy. Unfortunately the boy and his ingenious program didn't exist. Quick-witted readers would have noticed that 'Polo Flair' was an anagram for 'April Fool.'