Here's an interesting video from boingboing:

Boing Boing presents a remix of "Synesthesia," a documentary directed by Jonathan Fowler about people whose senses blend, or mix. For instance: a synesthete might see colors when listening to music, or taste flavors when hearing a spoken word.

In this documentary, Dr. David Eagleman of Baylor College of Medicine explains this condition, and four synesthetes explain how they perceive the world.