Paul Lamere — the guy behind stuff like Dogstep and this musical map of the U.S. — created Autocanonizer, a web app that turns any song into a canon. A canon, as you may recall from grade school music class, is any song that’s designed to be played against itself. Popular examples include tunes as grand as Pachelbel’s endlessly reused Canon in D and as simple as “Row, Row, Row Your Boat” (one basic type of canon is also known as a “round”).
The Autocanonizer analyzes whatever song you give it to find the parts that sound best in harmony with each other, then creates a new version in which those parts happen at the same time. Lamere’s mournful autocanonized version of Adele’s “Someone Like You” is gives a good example of what the effect sounds like.
Because the software is essentially playing two copies of a song at the same time, it’s helpful to use repetitive recordings with sparse instrumentation to avoid any sonic overload when they’re doubled. My most successful autocanonization came from a song that’s basically a canon already: Spiritualized’s “Ladies and Gentlemen, We Are Floating in Space.”