Ontologies are becoming critical to e-Science, but there are challenges for the community to find the ontologies relevant to their work, to declare mappings among related ontologies, and to provide feedback or critiques to ontology developers. We have created BioPortal, a Web portal to a virtual library of ontologies on the Semantic Web and a tool set enabling the community to access, critique, and improve ontologies. The BioPortal library contains over 50 ontologies from the biological and medical domains. In addition to a Web interface enabling researchers in cyberspace to locate these knowledge resources, BioPortal provides a suite of Web services, including ontology categorization, term search, graphical ontology visualization, and ontology version histories. As development of the knowledge resources in e-Science is increasingly collaborative, we are also creating novel tools in BioPortal to enable the community to create mappings between classes in related ontologies and to critique ontology content, providing feedback to ontology developers. Preliminary user experience with BioPortal has been extremely positive. BioPortal appears promising for unifying and disseminating ontology content on the Semantic Web, and it is providing tools needed by the research community to exploit these rich resources.