In the present post-genomic era, many annotation catalogs are being created for annotating biological objects, for example, structural families, functional annotations, or expression levels in microarray experiments. Given a set of biological objects, one of the most important biological research goals is to find meaningful correspondences between annotations from different catalogs. For catalogs with linear structures, e.g., real numbers like expression levels, this problem can be solved by using computational methods. However, for catalogs with complex structures, e.g., functional annotations organized in a directed acyclic graph structure, this problem may be complicated for computer scientists, because these structures usually involve biological domain knowledge. Thus, it is necessary to have an interface that can help biologists discover important biological knowledge from these complex structures. In this paper, we propose the Gene Ontology Browsing Utility (GOBU), a Java-based software program, for integrating biological annotation catalogs under an extendable architecture that uses the Gene Ontology and a user-defined hierarchy as two main catalogs. GOBU is an OpenSource project hosted on OSSF and available at http://gobu.OpenFoundry.org/.