Colors and Keys

Wikipedia describes synaesthesia as"a condition in which the stimulation of one sensory modality gives rise to an experience in another modality". In other words, there some of people who can "hear" colors or "see" sounds.

One of the more interesting nominees from the 2006 Webby Awards is a site called Color of Sound, where sounds such as animal noises, chords, and popular songs are played with a color chart shown from which you can select an appropriate color. From the site's front page:

When the two senses involved are sound and color, this phenomenon is no longer limited to synaesthetes: anyone, given a sound and time for contemplation, can ascribe color to that sound. Oftentimes, these choices will overlap from person to person.

The Color of Sound project will no doubt yield some interesting results.

Essential reading for synaesthetes: Christian Bök's Eunoia.
Another Eunoia site