My own experiments with fore-edge decoration have mostly involved marbling. I decided to pull together photos of some of the books I've done so that they are all in one place. As you can see, I like things to be matchy-matchy. Every time I marble a textblock, I always make a matching paper so that I can use it on the cover of the book.
Fore-edge decoration is not limited to marbling, though. There are many techniques including painting, speckling, gilding, etc.
There are some really interesting examples of edge decoration on the Princeton University library website, using many different techniques. There is also a good article on Wikipedia about fore-edge painting.
