Soviet Union Prank
Wired magazine has an interesting article featuring their picks for the 10 best April Fool's gags in the digital era.
One that might be very familiar to truly old-school Usenet veterans is the launch of "kremvax", the Soviet Union's very own Usenet server, in 1984.
Of course, it was all a joke, but at the height of the Cold War, users across the globe bought right into the deception — a sure sign of a great April Fool's Day prank.
You can read a transcription of the original article as well as some reactions once the ruse was up at http://www.godfatherof.nl/kremvax.html.
Ironically, a few years later, a legitimate Usenet server actually did come online in Moscow, and many readers were skeptical that posts coming from the server weren't actually more pranks. Eventually, the server's administrators took the joke to heart and had their gateway renamed to kremvax.
Birth of SPAM
April 1 also marks another momentous event for Usenet. Unfortunately, this one has little to do with humor and much to do with the universal annoyance we call spam, and why we even call it 'spam' in the first place.
On March 31, 1993, a program called ARMM (Automated Retroactive Minimal Moderation), which was designed to streamline and automate the process of sending cancel messages for abusive posts, experienced a catastrophic bug. ARMM began posting follow-ups to its own messages, causing a recursive 'feedback loop' and flooding the news.admin.policy newsgroup with useless posts.
Joel Furr, then of considerable Usenet fame, described the flood of messages as 'spam' and the name simply stuck. Furr's usage of the term came from a popular Monty Python sketch in which Spam was the only item on a restaurant's menu, but no customers wanted. Accounts are varied, but most agree that Furr was the first to use the term to describe Usenet messages, and his usage of the term eventually migrated to unsolicited email.
Check out the Wikipedia article about spam for more history of the term.