IGN has a great feature up detailing the evolution of Batman's greatest foe- the grinning homicidal lunatic known simply as The Joker. It covers all the ground from his 1950's origins right through to the character's dark reimaginings in the upcoming The Dark Knight. I didn't think Heath Ledger could look particularly scary, but there you go.
Anyways, well worth the read. Fascinating to trace the development of a truly great villain.
Photo: Heath Ledger as The Joker. Courtesy of IGN.