Big Muff tone control
The passive "Big Muff tone control", originating from Electro-Harmonix's Big Muff Pi, has become a popular circuit building block, mainly due to its simplicity and versatility.
Standard circuit
The standard BMtc circuit doesn't have a flat frequency response in its "neutral" position. This is what the frequency response looks like when the potentiometer's centered.

Schematic

Wiring diagram
AMZ tone control
This improved version of the BMtc offers a second potentiometer, "Body", which controls the depth of the mid frequency notch, effectively being able to achieve a nearly linear response.

Schematic

Wiring diagram