Most of my personal work (beginning with my MFA thesis study) is based on my experiences as well as reading about synesthesia and hearing about how other synesthetes experience their world. Synesthesia is the consistent, involuntary combining of perceptual modalities sensory, cognitive, etc.) which occurs most often in the mind's eye. It is not a disease, disorder, hindrance, or problem; it is a fantastic phenomenon! It is difficult to determine the percentage of the population that is synesthetic, and it is possible that we are all born with multi-modal brains but unlearn them as we are taught the difference between sight, sound, etc.
Synesthesia takes many forms and is entirely unique to each individual. Some have colored alphabets, where, for example, the letter A is as much green as it is the letter A. The same can take place with numbers, days of the week, musical tones, and more. The study and interest in synesthesia (both in metaphoric and authentic forms) has seen an increase in the last decade, including an open dialog of neurologists, psychologists, writers, artists and more. Jump to the bottom of this page for suggested further reading!
Here are some links to the sites of fabulous members of our community.
University of Sussex (UK) has long been one of the pioneers of research.
The American Synesthesia Association has held several fascinating conferences, valuing artists, writers, neurologists, psychologists, and accounts from synesthetes.
The Canadian Synesthesia Association has collaborated on a couple conferences in our global community, and holds meetups.
The UK Synesthesia Association are our bloody brilliant counterparts across the pond.
The International Association of Synesthetes, Artists, and Scientists is a new organization that has sparked collaboration between the older, country-specific associations.
Sean Day, one of the founding leaders of the IASAS, as a personal page with an abundance of information and links.
The synesthesia Wikipedia page is fairly well monitored, with many great references.