Natto is fermented soybeans. They look kind of like little peanuts stuck together in gooey peanut butter, but with a soft texture and a, er, unique taste which most people either love or hate. The smell (a funky old-socks kind of smell) puts many people off. But it's supposedly really good for you.

Personally, I love natto with soy sauce, but wouldn't eat it on its own.