Do you enjoy taking pictures?
If you are any kind of photographer then you can generate passive income simply by uploading photos to a stock photo site. Generally stock photo sites take a certain percentage of the sales you make but they will do all of the advertising for you and will be able to give you a much larger reach than most are able to get on their own. Depending on the sites you may be able to signup with several to have a bigger audience.
A 216 page guide to selling your photos. http://typinginthecash.com/a/photos1/
A comprehensive guide to selling stock photos online. http://typinginthecash.com/a/photos2/
Paid For Pictures teaches you how to sell stock photos online. http://typinginthecash.com/a/photos3/
An excellent guide to help you enter the stock photo business. http://typinginthecash.com/a/photos4/