Not really making a mainstream appearance until extreme metal band Meshuggah adopted them in the early 2000's, 8-string guitars are becoming ever more popular. These behemoths are a standard guitar with two lower bass strings; with a neck the size of a 2 x 4, they really are intimidating. However they are cutting a swath through the world of music by opening the sonic landscape to some unique and profound musical expression all allowed by the impressive range of the instrument.
Essential listening would be Charlie Hunter, Animals as Leaders, and the Deftones - all artists that use these instruments almost exclusively. Noted for their incredible and earth-shaking low end, 8 strings are loved by heavy metal musicians everywhere. More surprisingly though is their new found acceptance amongst musicians in jazz circles, as the harmonic possibilities of 2 extra strings is just too delicious to pass up.
Many respected guitar companies have caught on and started manufacturing the instruments, most notably Ibanez, Schecter, and ESP. Like any instrument, you get what you pay for. There are some quite respectable starter models in the lower end of the price spectrum, usually between three and five hundred dollars. But for the more serious sonic explorer, be prepared to shell out upwards of two thousand greenbacks for one of these babies. Luckily, the reviews are all quite positive with most new players becoming hooked and never looking back to their old, outdated six strings.
Definitely an essential addition to any adventurous player's lineup, the 8-string is here to stay and play its part in the evolution of the instrument. Below are some great links to learn more and see it in action: