NUMBER 1: Performers who make nearly all the sounds with their voices and, perhaps, a single instrument.
Typically, there is some looping and/or beatboxing involved as is the case with comedian Reggie Watts, whose 2009 performance of "I Just Want To" alone has been seen over one million times:
Yancarlos Sanchez's Kalimba performance video -- alternately known as the "one man jazz band" -- was posted in 2008 and still draws views:
And finally, perhaps the best known on YouTube is Greg Patillo, whose beatboxing flute videos have been seen tens of millions of times. Here he performs, with his voice, flute, AND piano:
There are lots of examples of these that you might have seen (And it's not just guys that do this stuff), so feel free to share any others.
And check back soon for more on other two forms of popular YouTube "one man bands."