1965 - BBC

Smell-o-vision: In 1965, the BBC played an April Fool's Joke - the network aired an interview with a man who claimed that viewers at home could experience aromas produced in a TV studio. They went on to demonstrate by cutting onions and brewing coffee, then had "viewers" call in with claims they could smell these scents, thus convincing the viewers it was true. Amazingly, they are close to actually implementing this technology!
see Internet Smellovision by 2012?

Another attempt and what could have contributed to the idea is a gimmick created by Mike Todd, Jr - Smell-O-Vision

1998 Alabama Legislature Lays Siege to Pi

1998 April fools joke that was published in the Associated Press April 1 stirred quite the controversy. They reported that a newsletter of New Mexicans for Science and Reason contained an article written by physicist Mark Boslough stating that the Alabama Legislature voted to change the value of the mathematical constant pi to the decimal equivalent of the “Biblical value” or, the value of 3.0. The claim originated from a headline that appeared first as a news story in the 1961 science fiction novel called Stranger in a Strange Land written by Robert A. Heinlein. Further iterations and mutated versions of this story lingers today, most recently was a follow up story in 2007 - it's headline read "Rest Easy! Pi Is Not Changing!"
Source: http://www.nmsr.org/alabama.htm

2007 and Forward - April Fool Web sites

It started to take shape, people were making fake web sites for April Fools day. One noted was created by Dan Baines who described with great detail, illustrations, and pictures the discovery of a real Fairy. He further supported his claim with pictures of a mummified corpse that was found by a hound along a Roman roadway in Derbyshire, England. The bones were described as being light weight and hallow, allowing for this bird-like human to fly and attributed it's features as leaf like or "lifelike" wings. His site received hundreds of comments and letters from concerned fairly-loving readers to which he had to fess up - it was a prank.
source: ABC News: Top 10 April Fools' Day Joke Web Sites

2005 - Opera's SoundWave Technology

On April 1, 2005, Opera issued a press release announcing 'Opera SoundWave technology', as they described "Opera's patent-pending P2P speech technology uses analogue signals carried through open air, enabling users to communicate in real- time without the use of computers or mobile phones".

During this time in the browser-wars between Opera, Mozilla, Microsoft, this statement didn't seem so out in left field. They further claimed that this technology "solves the problem of frequent misunderstandings in non-contextual communication. Opera researchers realized this technology can incorporate dynamic emoticons far more advanced than those typically used in e-mails and instant messaging." and "Furthermore, the technology does not seem to work well over distances of more than 100 feet, and several issues related to security and privacy remain to be fixed."

Their press-release stated that the company had 'accidentally' discovered SoundWave during an R&D study to enable speech technologies in Opera's e-mail client, and also included a link to a demo of the analog-signal-processing technology. We all hoped that continued healthy competition in the browser market would prompt further advances in Web-enabled communications, but as time told, Opera's 2006 venture into stock photography was not exactly what they needed.
source: Opera Software Press Release

Bill Gates Dead:

2003 Bill Gates is dead, shot by a lone gunman at a charity event in Los Angeles. After three South Korean networks broadcast the story on local TV, ensuing panic triggers a 1.5 percent drop in the Seoul stock exchange — a value loss of $3 billion. Just another Windows-related crash.

See TELNET Subliminal-Message Option

Read more Classic April Fool Jokes