e4Graph is a C++ library and a Tcl binding providing persistent, reliable, cross-platform storage for graph like structures. Please visit http://e4graph.sourceforge.net for more info.
Metakit is an efficient database library with a small footprint. It's a cross between flat-file, RDBMS, and OODBMS.
Keywords: structured storage, transacted, load on-demand, portable, C++, Python, Tcl, instant schema versioning.