Synaesthesia is definitely one of the most fascinating things in the world: the mixing of the senses. A person who can taste colors or see sounds is known as a "synesthete", and although it can sometimes be artificially induced by LSD or the like it can happen naturally...Olivier Messiaen, for example, heard colors. Composer Michael Torke "perceives colors for various time units".
That quote is from the amazing Wikipedia entry for synaesthsia:
Among other interesting tidbits, it claims that Aphex Twin (Richard D. James) is a synesthete, and that the physicist Richard Feynman saw "the algebraic symbols of Bessel functions in colour". Amazing.
I read an article once by someone claiming that almost everyone is a synesthete when we are born, but for most people the brain eventually learns to separate the stimuli into categories...
In any event. I just think it's interesting that so much of art is dedicated towards achieving this sort of effect - composers (including myself) always talk about "color" and "shape" sometimes even "taste". But only a tiny fraction of the population can actually mix their sensations like that. I wonder if art is the human instinct to get back to the time when we were just born and all of our senses were one big mush.