A great article by Jason Fried on Inc.com about making use of, and profiting from, the natural byproducts of your core business:

Just like the lumber industry can sell its sawdust (a byproduct of milling trees), we discovered that we could sell our knowledge (a byproduct of running a business). […]

Whenever you make something, you make something else. Your byproducts may not be as obvious as sawdust, but they’re there.