Crazy Go Nuts

[The Grumpy Celt]

Where can I find some useful games rules for insanity?

 

[Lonely Tylenol]

Unearthed Arcana's got some. Essentially the d20 Call of Cthulhu rules reprinted.

 

[Krieg]

The following all have rules for insanity:

CoC D20/Unearthed Arcana
Grim Tales
OGL Horror

 

[Aitch Eye]

I haven't read it, but there's the Book of Broken Dreams PDF.

http://www.rpgnow.com/product_info.php?products_id=157&

 

[Cthulhu's Librarian]

As the others have said, CoC d20 or Grim Tales. Those are the ones I'd recommend.