Pinterest is the hot new start-up that allows you to pin things to a board that users find interesting. The links are connected to an e-commerce website that has an affiliate program. However it appears that Pinterest is modifying the link to add their own affiliate tracking code.
If someone clicks on the link through the picture and makes a purchase, Pinterest gets paid. Currently Pinterest does not have a disclosure of this link modification. This is interesting because Pinterest is trying to monetize at such an early stage of growth. Pinterest is secretly modifying links without proper disclosure either.
Here is an example of Pinterest modifying @free’s links on a pin:
Pinterest can modify tons of links at a time by using a service called skimlinks. Skimlinks is able to go through a site and add affiliate links wherever there is a link to a product that has an affiliate program associated with it. Many websites like Metafilter has taken advantage of the service.