Common among artists, Synesthesia is a cross wiring of the senses that allows the Synesthete to experience one sense (for example colour) in conjuntion with another (such as taste or sound). For example, a Synesthete might describe the color red as being hot, brown as being slick, and blue as making a ringing sound akin to a telephone, another might describe the sound of C Sharp as "tasting salty". Various explanations exist for the condition.
Famous synesthetes include Vladimir Nabokov (see his autobiography "Speak Memory"), Duke Ellington , David Hockney and Richard Feynmann.
Further links and videos are listed below.
Brief definition with examples at: Absolute Astronomy
Website for Synesthetes: www.synesthete.org
YouTube clip explaining: http://www.youtube.com/watch?v=DvwTSEwVBfc