For those of you who aren't familiar with the term, multiphonics refers to the production of two or more simultaneous pitches on a medium that is normally monophonic, or that normally produces only one tone.
Multiphonics are most popular in contemporary classical music, called for in some flute, trumpet, and trombone literature. It's easiest to get multiphonics out of medium/low brass instruments (especially horn), as the tones are produced by buzzing one pitch while singing or humming another, while woodwind players have to do all sorts of other crazy hoo-ha to make it work. In a brass instrument, this will sometimes result in more than 2 simultaneous pitches; through the magic of acoustics, anywhere between 2-4+ tones will be heard at one time, depending on the buzzed note, the fingering behind the buzzed note, and the note being sung. http://www.youtube.com/results?search_query=multiphonics&search_type=&aq=f
brings up several videos demonstrating multiphonics on various instruments. I would embed videos, but my internet is being teH gayZ and will not load any videos, so I cannot pick any to recommend, but there should be a fairly amazing video of someone playing tuba while beatboxing while doing multiphonics. Simply amazing.