Oracle Berkeley DB - Oracle Berkeley DB is an extensive embedded key-value database library that is providing support with scalability in performance for data management services and applications.
VistaDB - VistaDB is an all-in-one embedded database software for.Net that is completely compliant with ADO.NET providing support for the entity framework.
MySQL - The world's most popular open source database
SQLite - SQLite Home Page
PostgreSQL - PostgreSQL is a powerful, open source object-relational database system.
MongoDB - MongoDB (from "humongous") is a scalable, high-performance NoSQL database.