There are a number of so-called "micro-stock" sites out there. You get paid a small amount for each photo which someone downloads from the site which was submitted by you, usually from $0.20 to $1.00. Some people have gotten hundreds or even over $1000 a month doing this. This can be a great way to pay for this expensive "habit" that photographers have. Images are available for subscribers to purchase 24 hours a day, 7 days a week, so one can earn money while they are sleeping. The last time I remember being able to do something like that was when I tucked a baby tooth under the pillow for the Tooth Fairy to take and I'd find several quarters under the pillow in the morning. That being said, I've probably sold the equivalent of many jaw-fulls of teeth at this point.
The pictures you submit must meet certain technical and artistic criteria and pass the muster of a reviewer employed by the site. Another major part of submitting pictures is that it is critical to choose proper keywords to add to your photos so that they may be found by clients who are looking for a particular item, for example, halloween or pepper, in the search engine used by the site.
While I've made my current thoughts on microstock clear, there is a competing school of thought which takes issue with and opposes microstock. These are the more traditional Rights-Managed or Royalty Stock photography as practiced by Getty and Corbis members. An illuminating and insightful article on the topic is here at CNN-Money which may be of interest.
In this gallery, I put up the photos that have been accepted as stock by at least one of the sites.
Currently, I send to: