Mentos Turns Coke/Mentos Geysers Into Video Contest – this article says it as well as I could, so I’m not going to repeat it. But I suppose you need to know what it is, so here’s a quick summary.
Put Mentos in Coke. Booooooosh. Quite funny. People video it. Youtube. Phenomenon. Mentos respond. Coke don’t.
Also featured on Random Culture with some good links.