Google has been a pioneer since it's existence and as far as I can tell they have lead the way with their annual April Fools day announcements that seem almost plausible and have left more than a few people doing double takes. This years pranks are no exception to the list of far out ideas, though I think the Google Moon base job opportunity of 2004 is probably my favorite. This year it's, amongst other things, an 8-bit map and an autonomous Nascar.
For the yearly wrap-up check out this wikipedia entry.
Oh, and not to be out done, Sir Richard Branson just announced via twitter (@richardbranson) that Virgin is launching the UK's first naked workout sessions and trips to the earth's core via the 5 most active volcanos "Introducing
#VirginVolcanic, journeying to the centre of the earth thru the world’s 5 most active volcanoes! ".
Please leave a comment if you've discovered any others that you'd like to share!