UPDATE: If you're as April Fools'-obsessed as we are, you've probably spent the entire day trolling the Internet for pranks. And because so many big-name brands stepped up, we all had a lot to enjoy. (See our detailed list near the bottom of this post.) But a few gags rose above the rest. Topping Fast Company's totally informal ranking: The Guardian announcing  it will publish exclusively in 140-character tweets; Gmail launching an AutoPilot feature ; and Qualcomm revealing  it uses "wolfpidgeons" to transmit wireless signals. Congrats, guys. You all deserve gilded Whoopee cushions.
Ashton Kutcher might have coined the term, but it was Richard Dimbleby--as in, the usually somber BBC news anchor--who pioneered the “punk.” On April 1, 1957, during his widely respected news program, Panorama, Dimbleby voiced a two-minute segment on “spaghetti harvesting” in Switzerland (right). As he championed the practice, viewers watched “real” footage of spaghetti farmers pulling pasta from trees. “There’s nothing like real, home-grown spaghetti,“ he concluded.
The footage, of course, was fake. But its impact was very real: Hundreds of viewers called the BBC, wanting to know how they could grow their own spaghetti trees. (The network’s response: “Take a sprig of pasta, place it in tomato sauce, and wait.”) Today, the gag remains one of the greatest corporate stunts of all time, according to the Museum of Hoaxes .
Since the BBC broadcast, many big-name brands, like Google, Microsoft and Burger King, have played their own April Fools’ Day pranks. To consumers, the gags seem like fun and games. Yet Richard Laermer, CEO of RLM Public Relations  and author of Punk Marketing: Get Off Your Ass and Join the Revolution, says they’re an invaluable marketing opportunity. “Many people think of big companies as cogs in a wheel,” he explains. “A clever April Fools’ Day prank”--like Google Australia’s gDay , which promised to search Web sites 24 hours before they were created--“is a great way to change that.” This holds especially true during a recession, when many people are "desperate" for laughter.
But George Silverman, the Market Navigation  president who penned The Secrets of Word of Mouth Marketing, urges struggling companies to re-think their hijinks. The same gag that earns raves for "fun, innovative" Google and Facebook, he says, could backfire on banks and automakers. "Imagine if General Motors or Ford pulled an April Fools' Day prank," he says. "You probably wouldn't laugh. You'd be too busy wondering why they were joking around instead of making better cars."
As April 1 looms, there’s no telling what, exactly, Corporate America has planned. If it’s taking cues from years past, however, we’re in for some pretty epic pranks. Here’s a look at ten classics, and -- as we discover them -- the cream of today's crop:
- On April 1, 1996, Taco Bell announced  that it had purchased the Liberty Bell, which would accordingly become the "Taco Liberty Bell." When asked about the sale, then-White House press secretary Mike McCurry joked that the Lincoln Memorial had also been sold, and would now be the “Ford Lincoln Mercury Memorial.”
- On April 1, 1998, Burger King started promoting  a Left-Handed Whopper, whose condiments were “designed” to drip out the right side. According to reports, several eager lefties tried ordering the burger--presumably before checking their calendars.
- On April 1, 1998, Guinness announced  that it would be the official beer sponsor of the Old Royal Observatory’s millennium celebration. As part of the deal, “Greenwich Mean Time” would become “Guinness Mean Time” until the end of 1999. Obviously missing the joke, The Financial Times scolded Guinness for setting a "brash tone for the millennium." (It later printed a retraction.)
- On April 1, 2000, Google revealed its new MentalPlex  tech, which could read a user’s mind to determine his search query--before he even touched the keyboard. Alas, all results were April Fools’-related.
- On April 1, 2002, the British supermarket chain Tesco advertised  a new, genetically modified “whistling carrot.” The veggie had apparently been “specially engineered” to grow with “tapered airholes,” which made it whistle while it cooked.
- On April 1, 2004, National Public Radio's “All Things Considered” reported  on the Post Office’s new portable “Portable Zip Codes” program, which would allow people to keep their zip codes no matter where they moved. In the fictional report, an “official” source said the effort would “serve as an umbilical cord to the places [people] love best.”
- On April 1, 2005, the NASA Web site featured a news story touting pictures of water on Mars. Revelation? Not quite: The actual photo  showed a glass of water resting on a Mars candy bar.
- On April 1, 2008, Gmail started promoting Custom Time , which allowed users to adjust email time stamps. The science behind it apparently involved an “e-flux capacitor.”
- On April 1, 2008, YouTube linked every video on its homepage to Rick Astley’s “Never Gonna Give You Up.”  And yes, there’s a verb for that: “RickRolling.”
- On April 1, 2008, an executive at Microsoft Research said  his team had determined the true value of pi to be “a definitive 3.141999, or as expressed in company literature, ‘Three easy payments of 1.047333.’”
And today's highlights:
- Reddit redesigned its homepage  to mimic Digg's. There's even an option to "redd" your favorite postings.
- YouTube turned its videos  upside-down.
- Gmail launched an AutoPilot feature , which "analyzes" your personal messages, and responds (and offends) all by itself.
- Fark launched its own Facebook .
- The Guardian will now publish exclusively  in 140-character tweets.
- Amazon is making clouds .
- Qualcomm revealed it uses "wolfpidgeons"  to transmit wireless signals.
Think we missed a prank? Feel free to add more in the comments. Or, you know, just play one on a coworker.