SQLite

SQLite is the open source database format used for messages.dat. It is a type of file based SQL database, that runs without a server.

Features
Some features worth mentioning:
 * Supports most common SQL commands.
 * All tables are stored in a single file.
 * No server required.
 * Open format.

More Information
More Informations are available at the SQLite home page. The structure of the messages.dat file can be found here. The SQLite Database Browser can be used to view and edit the database.