Round mints. They used to have these really lame commercials back in the 90's, with people with miniscule problems that were solved, and had a mentos to celebrate. [See one of them]. Foo Fighters even made a spoof of them in one of their videos [clicky]
Anyway, this is pretty old. Apparently it happens because there are so many tiny little holes and ridges in the texture of mentos, it creates an explosion of bubbles.

