Blizzard http://www.worldofwarcraft.com/index.xml
Triple hit of new "hero class" (Bard) announcement, Warcraft for Consoles announcement, and (my favourite of the 3) a client side script on their forums that litter everyone's post with "lols" and other forumisms.
Assasin's Creed vs. Metal Gear http://www.youtube.com/watch?v=cxw91PSTRq8
Cute piece of cross-promotion. Not sure if it really counts as an April Fool, since the Assassin costume will probably end up being an unlockable.
Websites redirecting to other websites (ie C64 site http://www.c64.sk/ redirected to a Spectrum site). Oh the hilarity. Unless you actually want to use the website today. At all.