Oracle Berkeley DB is a family of open source embeddable databases
that allows developers to incorporate within their applications a
fast, scalable, transactional database engine with industrial grade
reliability and availability. As a result, customers and end-users
will experience an application that simply works, reliably manages
data, can scale under extreme load, but requires no ongoing database
administration. As a developer, you can focus on your application and
be confident that Oracle Berkeley DB will manage your persistence
needs.

Note that Berkeley DB 6 changed license to the
Affero GNU General Public License v3 (AGPL v3).

WWW: http://www.oracle.com/us/products/database/berkeley-db/db/