Ken Varnum presents on MTagger, the University of Michigan Library's social bookmarking tool. MTagger allows users to tag and bookmark library resources. While initial usage was lower than expected, with most items tagged by a single user, usability studies revealed tagging purposes were unclear. Recommendations included improving tag display and handling collections. Future goals include emphasizing patron tagging and improving the interface for secondary discovery of materials. Open sharing of tags and code across libraries could provide more benefits of scale and focus found in other social bookmarking platforms.