These are the best beatboxers I've found on the Net until now. Simply awesome.
Frenchman Joseph The Human Beatboxer who competed in the French version of Idols:
Find more on him here
This dude (I don't know his name, if any1 does, please leave a message :)
And ofcourse Kenny Muhammed, to most he is the best there is. And they're probably right :)
Find more on him here
As for soundeffects, there's Michael Winslow :D
