Microsoft SQL Server Express 2005

From Microsoft:

SQL Server Express is a version of SQL Server 2005 designed for building simple, dynamic applications. It helps developers build robust and reliable applications by providing a powerful database that is also free and easy to use. Too often database systems are overly complex for building simple applications. This and the other Express products reduce this complexity by providing a simple but powerful development environment for building data-driven applications. You can design schemas, add data, and query local databases, all inside the Visual Studio 2005 environment. If you need more advanced database features, then SQL Server Express can be seamlessly upgraded to more sophisticated versions of SQL Server.