Today was (is?) April Fools. Quite a lot of good stuff today.
YouTube made this video even more awesome.
Slashdot added achievements (which will probably stick around)
Google's big show this year was impressively extensive. Although... remember when everyone thought that GMail was an April Fools joke? It did sound too good to be true...
Reddit had a pretty good sense of humor to make itself look like Digg for the day (a lot of content makes its way from Reddit to Digg, and vice versa).
For the end-all be-all rundown, checkout http://aprilfoolsdayontheweb.com/