The University of Michigan library launched MTagger, a social bookmarking tool, in the winter of 2008. MTagger allows users to tag webpage on the library site, catalog records, or digital images, or anywhere else. MTagger is deeply integrated into our VuFind experimental catalog (launched February 2009), providing the mechanism for users to select and sort their “Favorites.” It is also part of our new Drupal web site (launched August 2009). MTagger preserves the concept of “collections” and enables a faceted approach for users to narrow search results. The tool was intended to enhance findability across collections and to expose “hidden” collections. Learn about the library’s original design, how we conducted usability testing, what we found, and how we changed the application in response.