Yerba Mate [wikipedia.org]
Got introduced to it when I was living in Argentina during the '70's. You could get it in the States from places like Pier 1 and other places that imported specialty foods from South America. It has been showing up in some grocery stores as a "sports drink" during the last few years but it's originally served steeped like tea and that is how I still drink it.
Though the "Orange Exuberance" flavor of the canned stuff is really good too and I try to keep a few cans around for when I'm too tired to make it from scratch and need a fast caffeine hit.