We all know what day it is today, and the Internet does, too. Let's see all the pranks we can find!
xkcd - Has done a prank every year, but today's is pretty impressive, although not as funny as the text-based parser imput (you could pet a kitty!) or the crossover they did with Dinosaur Comics. Randall turned all his comics into headache-inducing 3-D ones.
Jay Is Games - Last year, they tricked thousands into thinking there was a hidden game on the Zombocom website. This year, they reviewed a flash version of Tic-Tac-Toe as if it were a high-stakes RTS war game. Good times.
Found any more pranks?