hybridmoment
This website is one of many experimental art things that Rafael Rozendaal came up with and Reinier Feijen coded (according to a right click). Click the icons on the top bar of the first linked website, or go here to see the rest of their work. Each piece is sort of a short, interactive combination of color and sound. If you have epilepsy, beware.
2012 update with newer favorites:
goodbyefarewell.com
intotime.us
likethisforever.com
[…] This is similar, but runs in a browser and does not involve music. […]