Cry, Baby....

The wah-wah pedal effect for the guitar, now popularly referred to as the "Cry Baby Wah" thanks to the Thomas Organ company who commercialized the funky foot-actuated device in 1966, can actually be traced back to the efforts of Chet Atkins in the late 1950's:

Purists will note that Atkins' "wah" effect is actually the result of creatively adjusting the tone and volume controls on the guitar itself, while the true wah pedal of the 60's would rely on *semi* sophisticated electronics to modify the instrument's tonal character.
Non-purists will note that the mechanics of how the effect is created are irrelevant.