The first master of the Wah Wah guitar sound was Earl Hooker--before the Wah Wah pedal was invented. He used the volume control on his guitar:
The Wah Wah pedal was invented by accident while making a new kind of amplifier at Vox/Thomas Organ Co. in 1966. If you like stories about circuit boards, click here.
Some of the first guitarists to use it were Jimi Hendrix and Eric Clapton. Here' s Clapton in Cream's White Room:
It quickly moved to Motown and became associated with funk (and cop show music). Here is Isaac Hayes live with the Theme from Shaft:
Wah wah guitar also made its way into Boogaloo. Here is Dave Cortes:
Well, I don't know about you, but I feel all psychedelic after all the wah wah guitar. There is such a thing as too much wah wah, but I am not going to torture you and me with bad examples (esp. not me, because I have to find the examples). Let's just say you know there's too much wah wah when the music starts to sound really cheap--vinyl leather jacket cheap. And the audience starts to leave.
If you can think of any more very different examples of wah wah and how it's used--please comment or email and let me know. I'm always lookin', lookin' lookin' for somethin' I haven't heard before...