Stardog is a semantic graph database: fast, lightweight, pure Java for mission-critical, enterprise apps.
Pelorus is a faceted navigation tool for browsing, searching, and discovering information in complex, large data sets. Check out the cool demo.
Pellet is the leading open source OWL 2 reasoner. Check out the source, docs, and more at Github.
Spanner is an adjunct to Stardog that extracts structured information from documents, using NLP and machine learning.