HampusDB is a small, flexible and efficient hierarchical database. It comes with a wide support of command line utilities for manipulating and extracting data.
It's designed for both embedded and bigger systems. HampusDB currently runs on Linux and has interfaces to C, C , Java and Perl.
HDB aims to fill the gap when storing data in a relational database is too rigid and storing data in textfiles is too cumbersome.
A typical example would be when you have heirarchical data such as XML or configuration data that you want to store and retrieve in a flexible manner.