http://e4graph.sourceforge.net
e4graph is a C++ library for providing persistent storage for graph data.
[Explain.]
[Cool things: swappable, even scriptable, back end, at least architecturally. Arbitrary data stores. Conceptual power. Embeddability. Portability of data and source. Screaming performance. ...]
e4Graph makes a prominent appearance in a developerWorks article [L1 ] on high-performance XML (which subject, incidentally, ought also focus attention on tDOM.