Parka-DB (again?)

This is probably the closest product I've seen to using an Open Source RDF DB.

http://www.cs.umd.edu/projects/plus/Parka/parka-db.html

The MindSwap.org web site has an interesting page listing their current status on using it to store,
index and search on it.

http://www.mindswap.org/~rreck/pp_overview.shtml

The have a publicly available CVS repository:

http://iama.rrecktek.com/cgi-bin/cvsweb.cgi/