Charlie Hunter's guitar is an 8-string tuned E-A-D for the 3 bass strings and A-D-G-B-E for the top 5 guitar strings. They are custom Novax guitars made by guitar maker Ralph Novak (the guitar maker is spelled with a k, his company with an x). I have yet to hear of any other guitarist who can play nearly as competently as Hunter with a similar dual-range guitar.
Classical guitarist Paul Galbraith also plays an 8-string guitar (the "Brahms guitar," so named because he had it designed for himself while working on a Brahms piece), but it is tuned like a conventional guitar with added lower and higher strings.
Not to be confused with seven string guitars, which have recently become quite popular with the nu-metal crowd, although a few famous old jazzers like George Van Eps and Bucky Pizzarelli have used them to great effect over the years. They are (usually) simply 6-string guitars with an added low-B string.