Clock

Technically a clock is just an oscillator - usually one that produces a pulse wave, generally at subsonic frequencies. It is commonly used to provide a timer for triggering envelopes, Sample & hold or sequencing modules. Some clock sources will go to low frequency audio or even higher rates, allowing them to be used as a sync or reset source for audio oscillators, LFOs and cycling modules.

Feel free to add to this list. See also: Modules by Function