BARNACLES! I know. It seems strange. But they do. The largest penis in the world! (Relative to body size that is… The actual largest penis belongs to *SHOCKER!* the largest animal, the blue whale.) Barnacle penises are up to 8 times their body size. In human terms, this would be like a 6 foot tall man having 48 foot long penis!
You may be wondering… Why do barnacles have such long penises? Well the answer is simple… SEX! Okay, so maybe I should explain. Barnacles are sessile, meaning they don’t move. Could you imagine trying to have sex without moving? It’d be quite difficult! Barnacles are hermaphroditic and lay on their backs, so they have extra long penises to reach across to their neighbor and fertilize their neighbor’s eggs. In return, their neighbors (sometimes multiple) fertilize their eggs. And what about those lonely barnacles all by their lonesome? Well they fertilize themselves. Power to the species!
More Links:
http://www.sciencedaily.com/releases/2008/02/080206150703.htm
http://news.nationalgeographic.com/news/2008/02/080213-barnacle-penis.html
http://www.newscientist.com/article/dn16971-penis-length-isnt-everything–for-barnacle-males.html
mothernaturespen 
Heh… it invites so many jokes, but really, I think the best thing about the Barnacle Story is that it puts guys squarely in their place.
I don’t envy them though, it must be boring being totally unable to move!