Paulstretch

Ordinary time-stretching tries to preserve rhythm and transients. Paulstretch, written by Romanian developer Nasca Octavian Paul in 2006, gives up on that deliberately. The audio is cut into long overlapping windows, each window is FFT-transformed, the phases are randomised, and the result is overlap-added back together. What survives is the colour of the spectrum without any time structure — roughly the sound of a choir echoing round a cathedral.

It became famous in 2010 when a Justin Bieber song slowed down 800% went viral and was widely declared better than the original. Today it ships as an effect in Audacity and underpins a great deal of ambient music. It is hopeless for anything that needs to keep its groove, which is rather the point.