MetaKit

Open Source Embedded Data Base library. It fills the gap between flat-file, relational, object-oriented, and tree-structured databases, supporting relational joins, serialization, nested structures, and instant schema evolution.

http://www.equi4.com/metakit/

There's a Python binding for MetaKit.

There's no full-text Search Engine for MetaKit, but Paul Snively thinks it shouldn't be too hard to add.

There's a library on top of MetaKit called [E4Graph] that implements storage for graph-structured data (a superset of tree-structured)