ThingDB

built in Python on top of PostgreSQL

http://simonwillison.net/2007/Jul/17/about/ - I Commented How does this differ from other triple stores? (but got no answer)

Another extensible Key-Value pair data store, constructed for the Open Library based on Aaron Swartz's InfoGami technology.
http://simonwillison.net/tags/thingdb
Mar'2013: by now, it probably makes more sense to use a more-popular NoSQL engine.
- actually, if you're trying to make it easier for non-techies to host, maybe not, since Hosted Server companies are likely to have MySQL or such in their default builds.

https://github.com/internetarchive/openlibrary

Bill Seitz