We describe tools in development at the University of Pennsylvania to generate and display interactive "subject maps" for exploring library collections. Based on the Library of Congress Subject Headings (LCSH), these maps are automatically built from existing authority records, a collection's bibliographic records, and optional local "tweaks" for local interests and search patterns. Users can explore these maps via ordinary text- based web browsing, and browse clusters of related research resources. We now provide these maps for small collections like The Online Books Page, and are experimenting with maps for the entire Penn Library catalog. We hope to enable users to take full advantage of the rich conceptual relationships in LCSH-based library collections, and effectively browse increasingly diverse and dispersed library collections.