http://www.youtube.com/watch?v=CNm8ZCJ7Fx8 (1911 Viral Videos)
http://armorgames.com/ (Bieber Fever)
http://www.thinkgeek.com/interests/looflirpa/ (Angry Birds Pork Rinds, Playmobil Apple Store, ...)

I'm sure more will appear soon.

Edit:

http://gmail.com/motion (Gmail Motion)

[Any weather report for Massachusetts, but this time it's not the 25" that fell 4/1/1997]

Edit 2:

http://chromeadblock.com/freedom/ ("augmented reality eyewear that detects and removes ads from the world in realtime")

Edit 3:
http://www.wholefoodsmarket.com/index0401.php (Whole Paycheck via ceo)

Edit 4:
CNN has an article with pointers to other April Fools web pages.
http://edition.cnn.com/2011/TECH/web/04/01/april.fools.online/index.html