RDF and Graph DBMSs A great page listing a lot of different types of databases:
* AKT Triplestore Used by Redland.
* Sesame Java based, B-Tree indexing.
* Kowari Java based, AVL Tree and Node/String pool. A good write up of the ideas is in Kowari: A Platform for Semantic Web Storage and Analysis and Yet Another RDF Store.
* YARS Available for download. Uses Java, JDBM and only 2MB.
* RDFStore C based, even has compressed triples. See Indexing and retrieving Semantic Web resources: the RDFStore model.
* SWI-Prolog/XPCE Semantic Web Library
* GRAS "A Graph-Oriented Database System for SE Applications"
I'm only including native RDF stores (non-SQL based).
As these are commercial only it's hard to tell what they're based on: