Beat Boxing

Beatboxing is making drum sounds with your mouth. It was never a huge part of hip hop, but was sort of big in the '80s (The Fat Boys, Doug E. Fresh, Biz Markie).

Here is Doug E. Fresh in 1986: http://www.youtube.com/watch?v=2ARBgBeHY1w
go to 0:59

It went out of style for a long time, but then some new people came along who took it even further.
Rahzel was a part of the act "The Roots." He innovated by incorporating the sounds of turntablism into the beatboxing.

Rahzel: http://www.youtube.com/watch?v=I9I6ir4WJN8&feature=related

Kenny Mohammad is known for the "wind technique," which involved simultaneous audible breathing while making beats with the mouth.

Kenny Mohammad: http://www.youtube.com/watch?v=D21CNg3Xwbs&feature=related

Two nights ago my man Jesse Stewart told me about Beardyman, and I'm very impressed.
He's incorporating technology and singing

Beardyman: http://www.youtube.com/watch?v=v2F-Emds6eA

Someone I don't hear associated with beatboxing much is Bobby McFerrin (of "Don't Worry, Be Happy" fame). Unlike the others, who focus on beats, McFerrin tries to make whole songs, including instruments, voice, and percussion, using beatboxing, singing, and percussion on his own body.

Bobby McFerrin: http://www.youtube.com/watch?v=BWu9v-1ROBA&feature=related

I don't think it's widely known that Michael Jackson could beatbox competently too:

Michael Jackson: http://www.youtube.com/watch?v=5wBvKOD4Zm0&feature=related

Here's a guy who beatboxes while playing the flute: http://www.youtube.com/watch?v=59ZX5qdIEB0&feature=related

If you're interested in beatboxing, there are how-to videos on youtube.