2005: Pandaren Xpress. This was the real beginning of Blizzard's silliness -- Sony had just recently announced a (real!) deal with Everquest and Pizza Hut, so the announcement by Blizzard that they'd made the same deal with a Chinese food chain sounded legit to many news outlets (including Slashdot), and got quite a bit of coverage for a joke. Blizzard must have smelled blood in the water, because after this, April Fool's Day was never the same for them.
2006: The first year that others picked up on Blizzard's jokes and started kicking off their own. The official joke was about Wisps as a playable race -- everyone was wondering just who the Alliance race was in the Burning Crusade (later revealed as the Draenei), and a lot of people bought the "Detonate" racial ability, though the "permanent death" feature was really the giveaway here. But the official joke wasn't the only one: Blizzard as a company gave some official credence to the BurgerCraft rumors, and Caydiem posted those (still brilliant) fake patch notes for the upcoming patch 1.11. And thanks to Gamespot, rumors of a "World of Starcraft" (along with a "leaked screenshot") also swirled around.
2007: And last year things went even further over the top. Blizzard announced a "new" RTS called Warcraft: Heroes of Azeroth ("WHOA," for short) "based in the WoW Universe," which was really the old Warcraft RTSes made to look like a WoW tie-in. The forums went a little nuts with "Blue for a Day," as every poster but the CMs got to go blue, also resulting in Tseric's classic "lol wut" remark (and making a few people unhappy). Unofficially, WoWWiki got turned into MeowWiki, Wowhead got sold to Blizzard (providing a weird foreshadowing for later that year), and a number of other Warcraft-related jokes flew around. But the big one was, of course, the Tinfoil Hat -- with the release of the Armory earlier in the year, lots of players were concerned that their gear choices were being shown off, so Blizzard released an engineering item that would supposedly keep your character out of the Armory (as well as dropping Intellect by 10 and preventing mind control... or maybe not). And they also poked fun at all of their attunements, with a hopelessly complicated attunement chart.
What's Blizzard got planned for this year? We haven't heard yet, and they haven't said. But whatever it is, if you're the gullible type, you might want to stay off the Internets tomorrow. Blizzard has a long tradition of pulling our legs right out of their sockets on April 1st. Get those grains of salt (and, please, that sense of humor) ready.