This so-called "un-dictionary" has apparently been around since mid-2009, but I only just discovered Wordnik today. It's UI is visually striking and easy to use, and it's community-driven, without being polluted and juvenile like Urban Dictionary.
The definition of "incendiary" on Wordnik.com
Detail of a word entry on Wordnik.com
Intelligence without ambition is a bird without wings. Salvador Dalí