Open Source - Databases
http://www.sqlite.org/
SQLite is a small C library that implements a self-contained, embeddable, zero-configuration SQL database engine.
http://monetdb.cwi.nl/
MonetDB is an open source high-performance database system developed at CWI, the Institute for Mathematics and Computer Science Research of The Netherlands.
http://www.ittia.com/
ITTIA offers database solutions for platforms where limited memory, storage, and processing power make software development a challenge. ITTIA develops fast, high performance database products and tools with the goal of providing excellent data management software for embedded devices.
http://www.kexi-project.org/
Kexi is a Free/Libre and Open-Source integrated data management application, a long awaited Open Source competitor for products like Microsoft Access. Kexi can be used for creating database schemas, inserting data, performing queries, and processing data.
http://www.tdbengine.org/
Tdbengine is a "relational database management system" - in short: RDBMS - with an integrated programming language. It represents the enhancement of the famous DOS-TDB and is predestined to handle databases on the web. It connects to the webserver using the standard cgi interface.