(I know, I know, but someone had to post this at some point. Figure it might as well be me.)
Author Eveline Chao and illustrator Chris Murphy serve up some naughty, naughty Mandarin in Niubi: The Real Chinese You Were Never Taught in School. 160 pages of handy up-to-date profanity and text that includes Chinese characters, pinyin, tonal marks, pronunciation guides and dubious etymologies. Fun for all ages.
ChinaSmack has good scans of text and illustrations from the book, and spirited Chinese and English commentary from readers.
Here's the Amazon link, and a link to an old Paper Republic discussion thread in which we hashed and mashed "The Unspeakable Bi".