I did my own quick survey to see how many applications and tools are using Jena: MIES, JemBlog, Ontolog, DSpace, Joseki, Euler proof mechanism (for just RDF/XML to N3 triple generation), RDF Grapher, EOR, IsaViz, RDFAuthor, DAMLJessKB, IdeaGraph, Ontology Middleware Module, Semantic Web Engine, RDF Topic Maps, TRIPLE, FenFire, DAML+OIL Plugin for Protege-2000, MDF, Sesame, and DELI (used for CC/PP in Cocoon).