A few sites are now offering users a way to access their content through open application programming interfaces (APIs). APIs are generally accessible via two types of technologies: web services and REST.

Web services
Amazon has been offering access to its data and content for several years now using web services. In fact, it has even more information available using its web services than what is displayed on its own website. For instance, while you can get the current sales rank for a book on Amazon.com, TitleZ.com uses Amazon web services to display the sales rank history for the last 7, 30, 90 days and lifetime.

REST APIs
Flickr has a full set of REST APIs that allow any developer to create new interfaces and display user-generated content -- in this case pictures -- in whatever way they want (assuming they are not doing anything evil with it). Flickr APIs provide users with the ability to search, upload, display and add comments to photos and much more without ever having to go to Flickr.com.

Several developers have experimented with these open APIs and have created very interesting and creative interfaces. Offering your users open APIs will spark creativity and create new applications that we can't yet imagine.