DigiTech Luxe Compact Polyphonic Detuner Pedal

They say:

If you're looking for a way to thicken up your tone without adding the weird warbling effect you get with a chorus or a flanger, then you've got to check out the DigiTech Luxe.

The Luxe is a brilliantly simple unison pedal that lets you blend a slightly detuned version of the input with the dry signal to create a constant thickening sound similar to the studio technique of double tracking. It works with all types of tones, from dry to heavily distorted, and it's really cool how the same effect can make both overdriven harmonics and bass guitar sound so much fatter.