A few years ago I registered the domain superfluousnipple.com, intending to make a shrine to that misunderstood phenomenon. I quickly realized that these guys beat me to it. This, combined with Wikipedia's page pretty much cover all the basic informetion you could ever need.
Anyway, in the other links, there is The Diaper Pail, which has nothing to do with superfluous nipples, oh no. It's all about using a fully functional pair of them for their intended purpose: feeding the cutest damn baby on the planet.
I better link to my other page too. Dread Pirate Robert has almost nothing to do with nipples, other than the writer has three.
So people, mail me some photos of third nipples! How about sending them to pics at superfluousnipple.com and I'll put them up here. Don't worry, Marky Mark makes me look bad too.