My favorite has always been http://xkcd.com/323/

Oh wow, how did I forget about little Bobby Tables. I was presented with that when I first got into web development (way back before I knew PHP) and was told to always check for that. "Don't end up the next Bobby Tables!"
saxon3049 said:
Damn it, every time there is a link to xkcd my productivity plummets for about 2 hours.

I hate sites like that....well I love them...but I hate them.... bah you get the point :p
Yea the black hole that absorbs all my time (also known as the internet) you find one of the true gems like xkcd and your lost for hours. It's almost like driving round Whales looks pretty and you can spend ages there but you will not get anything done.