Tuvan Throat Singers
Search for "Huun Huur Tu" on iTunes as well.
Very, very, very cool stuff. I used to have a CD of these guys with some didgeridoo accompaniment.
Interesting stuff. My mother and some associates used to use their music as background meditatory music in healing rituals. Throat music is though to be very therapeutic in some African and Native American shamanic tradition (this group in particular is Turkish) because of, I suppose, the assault of muti-tonal music. It's about vibrations, you see.
Some throat singers can project up to six notes at once.