Flickr

From Wikipedia:

Flickr is an image and video hosting website, web services suite, and online community platform. It was one of the earliest Web 2.0 applications. In addition to being a popular Web site for users to share personal photographs, the service is widely used by bloggers as a photo repository. Its popularity has been fuelled by its organization tools, which allow photos to be tagged and browsed by folksonomic means. As of November 2007, it hosts more than two billion images.
Flickr asks photo submitters to organize images using tags (a form of metadata), which allow searchers to find images concerning a certain topic such as place name or subject matter. Flickr was also an early website to implement tag clouds, which provide access to images tagged with the most popular keywords. Because of its support for tags, Flickr has been cited as a prime example of effective use of folksonomy, although Thomas Vander Wal suggested Flickr is not the best example.[14]
Flickr also allows users to organize their photos into "sets", or groups of photos that fall under the same heading. However, sets are more flexible than the traditional folder-based method of organizing files, as one photo can belong to one set, many sets, or none at all. Flickr's "sets", then, represent a form of categorical metadata rather than a physical hierarchy. Sets may be grouped into "collections", and collections further grouped into higher-order collections.
Finally, Flickr offers a fairly comprehensive web-service API that allows programmers to create applications that can perform almost any function a user on the Flickr site can do.[15]