http://www.cc.jyu.fi/~sjansson/throat.htm
an explanation of where it come from

http://www.plumebleue.ch/extrait/Diphonia_1.mp3
An mp3 but you can't really hear it, since there's another instrument

http://t.clements.free.fr/snd/TribeOfSound/TribeOfSound02.mp3
Pretty good one with a didgeridoo doing the rhythm section!

http://t.clements.free.fr/snd/TribeOfSound/TribeOfSound05.mp3
Another one by the same group, except this time time the harmonic stays the same while the actual singing changes.

It's also called "Diphonic singing"