One of the most impressive hacks I've ever read about has to be the Black Sunday kill. Since the original 2001 Slashdot article I read on this is 99.9% quote, I'm going to do the same. I can see why they quoted so extensively; it'd be difficult to improve on the unusually succinct, well written summary provided by Pat from Belch:
This is a companion discussion topic for the original blog entry at: http://www.codinghorror.com/blog/2008/05/revisiting-the-black-sunday-hack.html