I currently have 25 of my images up for sale at Getty Images. Getty chooses the images that they want to add to their collection of stock photography, and then the public can purchase rights to use these images for all different kinds of media. Getty takes a pretty big cut of the sale, but depending on the size of the image, as well as the exclusivity the buyer would like, it can be a fairly easy way to make some dough from photos that otherwise would gather dust on my hard drive. From what I've heard, it usually takes about 6 months for your images to start selling. It has been 5 months and I just made my first sale. The shot that sold was this one I made for Leap Day 2 years ago. Funny, huh? It sold for commercial web use in the beauty / personal care industry.
You can see my entire Getty collection here.