This is the Big Red Button.
There's just one rule: Do not press.
Thanks to the creator of the Big Green Button (the really old HTML one, not the newgrounds one) for the inspiration.
EDIT - August 2009 - To anyone who's mentionned they've seen this thing elsewhere, please note the upload date. I'm the original creator of the BRB and I never expected it would get so much "attention" if you could call it that. It's been copied over to nearly every "i'm bored" website and now there's even a god-damned iphone app for it! I'm both honoured and kind of upset that people are making money (however little) off my creation. Regardless, I'm glad everyone is still having fun with the little guy after 5 years. :D
EDIT - March 2010 - Because I'm seeing the question asked everywhere: The white button is on the second "no text" frame. It's in the very top-left corner. It originally linked to the Big Green Button (an html based game similar to mine - NOT the big green button on newgrounds) but that site has since stopped working.