Wah-wah pedal
A resonant band-pass filter whose centre frequency is swept by foot, making the guitar sound like a voice saying "wah".
Brad Plunkett at Warwick Electronics, owned by the Thomas Organ Company, built it in 1966 out of a midrange switch from a Vox amplifier. He replaced the switch with a potentiometer under a rocking treadle, and the result was sold as the Vox Clyde McCoy and later as the Cry Baby. The circuit is modest: one transistor, an inductor and a high-Q filter swept across the midrange.
Jimi Hendrix made it an instrument; 1970s funk and film soundtracks made it a cliché. Auto-wah, or envelope filter, lets playing dynamics move the filter instead, via an envelope follower. That the pedal still sells largely unchanged says something about how hard it is to improve on an inductor and a foot.