Any alternatives to Virtuoso as a graph store?

I like it (very much) that is supports SPARQL/Update and the SPARQL endpoint that comes with it, but

• I'm a little worried about vendor lock in
• I think it is overkill for my requirements (I want a graph store with half a billion triples)
• I would love to use an open-source and free product instead

So far I couldn't find any descent and comparable products (commercial or otherwise). They pretty much look immature or experimental to me. Ideas ?

What you might be looking for is http://4store.org/ and you might also try searching for questions very like this over on http://www.semanticoverflow.com/ (link is defunct)