I've become somewhat obsessed with Steve Reich's piece Piano Phase.
As I understand it, it's a work written for 2 pianos (though Reich has also given latitude for other instruments such as marimba, etc -- and has apparently said that he prefers this performed by percussionists rather than pianists). The pieces consists of twenty-three different musical ideas that are to be played for a duration as determined by the performers. They are instructed to begin playing each idea in unison, then one of the two should begin relaxing the tempo while they other keeps strict time so that the music begins "phasing" out of time, creating a particular kind of euphony that is intense.
Here's a performance with choreography that is breathtakingly and agonizingly beautiful.
There are times when I cannot be absolutely sure that the video speed is kept constant, such is the power of this performance to seemingly stop time. It feels that time itself is manipulated in this piece which is astounding.