I find the phenomenon of synaesthesia utterly fascinating. For those not familiar with it, synaesthesia is a class of related neurological...hmm..."disorders" isn't quite the right word...let's go with "phenomena" that have to do with the interplay between different sensory modalities. The most common manifestation of synaesthesia is grapheme-color synaesthesia, in which numbers or letters (and sometimes equations) are tied to particular visual experiences (e.g. color and/or shape). Many people with grapheme-color synaesthesia are able to make use of their condition, and find it very helpful when working with numbers and letters; and their ability to "translate" strings of characters into color-based experiences lets them memorize and work with those characters very easily.
Via the excellent (and aptly named) blog Neurophilosophy, I found out today about new research by Jamie Ward indicating that everyone has synaesthesia to some degree or another, but only some people are conscious of the connection between their modalities.
This is a visual rendition of the Wagner Opera Lohengrin, as depicted by synaesthete Wassily Kandinsky.
When I was in high school, we had to do science fair projects. One year, I did a project called "What Color Do You Hear?" In that project, I recorded a variety of common (and less common) sounds, played them for a variety of subjects, and asked them what color they would most closely associate with each sound. I got a few funny looks while doing it, but virtually no one said that they had no idea what I was talking about. This seems to lend credence to Ward's idea that everyone has a degree of synaesthesia.
Additionally, I've since learned that almost all synaesthetes with grapheme-color type synaesthesia "see" the letter 'A' as red. One of the sounds I played was the note 'A' (440 Hz) on a violin; it was overwhelmingly perceived as red. I wonder what the significance (if any) is of that?