HailDB is a relational database that is embeddable within applications. You embed HailDB by linking to a shared library and calling a clean and simple API. HailDB is a continuation of the Embedded InnoDB project. It is not itself a database server, but is a library implementing the storage layer. With the addition of the HailDB plugin to Drizzle you get a full SQL interface.
Read more at http://www.haildb.com