libdrizzle is the client and protocol library for the Drizzle project. The drizzled server will use this as the protocol library, as will the client utilities and any new projects that require low-level protocol communication (like proxies). It is designed to be used for other language interfaces such as PHP extensions and with SWIG.
HailDB is a relational database in shared library form that can be embedded into applications. It is not a SQL database, although you can use this library as the storage backend for a SQL database. It features a clean and simple API and is a continuation of the Embedded InnoDB project. With the addition of the HailDB plugin to Drizzle, you get a full SQL interface.