You'll want to take a look at this site: the URL is http://tags.library.upenn.edu
PennTags is an example of social bookmarking technology made into a research tool. PennTags can be used as a resource for storing, sharing, finding, and organizing research links.
Social networking sites like PennTags utilize the concept of a "Tag Cloud" to visually display tags that are used most often. Tags are not contolled, that is, people make up their own tags as descriptors of content that they find , but as the same tag is used by others, sites with related content become grouped together. Tags that are used most often are shown in larger font than those used less frequently.
If you would like to learn more about PennTags, questions or feedback, you can contact the PennTags group at penntags@pobox.upenn.edu.
