Octave effects and pitch shifter and harmonizer effects alter the pitch of the guitar's signal to create different types of harmonizing effects that can sound synthetic or unnatural from a normal guitar tone.
Octave Pedals
Octave effects combine the guitar's signal with a duplicated signal, raising or lowering it an octave in pitch, creating a synthesizer type of sound. Both guitarists and bassists use octave effects to create a lower range and thicken up their tone. Octave effects go hand-in-hand with fuzz and distortion and many of these pedals use the option of fuzz in the pedal controls.
Jimi Hedrix used a pedal called the Octavia, though he called it the Octavio, to record the solo heard on “Purple Haze”, and on the song “Fire” in 1967. The Octavia is an octave fuzz effects pedal invented by Roger Mayer, Hendrix’s sound tech, that became popular after Hendrix started playing through one. The version of the Octavia used on those recordings was never used again and the circuits and transformers were swapped out in the later versions that were eventually released.
The octave effects pedals available today combine the fuzz or overdrive and different options to create sounds that can make the electric guitar sound like a synthesizer or like a bass guitar.
Pitch Shifters & Harmonizers
Pitch Shifter effects alter the pitch of the guitar's original signal to create detuned or other pitch effects, and are not limited to just an octave up or down from the original note(s). Some pitch shifting effects can be used with an expression pedal to control the amount of the effect applied to the signal.
Harmonizer effects work similar to pitch shifters, and allow one guitar to sound like multiple guitars playing in harmony.