I organized one of my bookcases by spine color. It's a really easy way for me to find books because I can just think of a book and the spine color comes to mind. Furthermore, because the colors are grouped, my eye goes to the right section instantly.

This may not work for everybody, but I love the speed of access and the way my bookshelf looks as a result.

Edit: I have done this with my icons in the past, too: http://flickr.com/photos/porkfriedrice/3753844018/

I organize mine by size, so I have my own version of crazy.

There was a whole bookstore in S.F. that did this for a few weeks as an art project - http://www.superherolife.com/journal/archives/000453.html

I have organised mine by colour in the past also. It does impair discovery, but I tend to glance at my bookshelves more often than I hunt through them for a particular book.

Yeah. On the other hand, mine are disorganized and not easy to navigate except by me. They used to look like this: https://jseliger.wordpress.com/2009/09/07/bookshelves-office... and a photo of Neil Gaiman's are there too FWIW.