Synesthesia is a neurologically based condition in which stimulation of one sensory or cognitive pathway leads to automatic, involuntary experiences in a second sensory or cognitive pathway. People who report such experiences are known as synesthetes.

So, for example, people may involuntarily experience a colour whenever they see or think of a number. Many people with synesthesia use their experiences to aid in their creative process, and many non-synesthetes have attempted to create works of art that may capture what it is like to experience synesthesia.

http://colourfulanguage.wordpress.com/ is a blog by Lauren who lives in the UK, has synaesthesia, and likes it!