The Boss SD-1 is the longest running overdrive in the Boss range. Produced (and virtually unchanged) for over 20 years, the SD-1 is widely considered to be the asymmetrical clipping counterpart to the Ibanez Tubescreamer. The original Boss overdrive (the OD-1) lacked the frequency range of pedals like the Tubescreamer, so the SD-1 was a redesign of this circuit that included this function.  
The SD-1 features a Level knob that increases the output volume of the pedal and an Overdrive knob controls the intensity of the drive, along with a Tone knob to control the frequency.