Low Frequency Oscillator (LFO): is an electronic frequency which is usually below 20 Hz and creates a rhythmic pulse or sweep. They are used to create Vibrato, Tremolo and Phasing effects. First introduces in the 1960s for modular synthesizers, they are now used for samplers and software synthesizers to increase their sound alteration capabilities.
A LFO is a secondary oscillator that operates at a significantly lower frequency, typically below 20 Hz. This lower frequency or control signal is used to modulate another component’s value, changing the sound without introducing another source. Like a standard oscillator, this usually takes the form of a periodic waveform, such as a sine, saw-tooth, triangle or square wave. Tremolo, Vibrato and Phase pedals use low frequency oscillation but will not list it as a feature. The effect pedals labeled as a LFO effects let the user switch between the different wave forms.
Pedal Examples: 
Mastro Valvola LFO Optical Tremolo
Crazy Tube Circuits Stoned Hz V2
Adventure Audio Outer Rings Ring Modulator
EarthQuaker Devices Interstellar Orbiter Dual Resonant Filter
Fifty Favorite Effect Pedals for Sale: