This Banksy rat mural is possibly the best-documented one on flickr. But not to be outdone, I am uploading it to complete the set of four.
It shows a rat with a briefcase and an umbrella, and it reads "Let Them Eat Crack".
You can find this work of street art on Broadway and Howard St. It is to huge that it's practically impossible to miss.
Update 11/5/08: Looks like it's being painted over :(