I've always found this subject fascinating, and a lot of the art stemming from people who have had serious mental illnesses is amazing. A few examples:
Adolf Wölfli

Achilles Rizzoli (I have a book of this guy's work - incredible, fanciful stuff!)

Richard Dadd

Louis Wain - intriguing for his cat paintings. When he started (and was sane) his paintings looked like this:

but when madness took hold, they evolved into this:

Simon Rodia, builder of the Watts Towers
