MS SQL Server Express
This is free and it has most of the functionality of the full SQL Server product with limits on CPU, memory and Database size!!
SQL Server Express Edition helps developers build dynamic and reliable applications by providing a freely redistributable, easy to use, and robust database. It includes all the rich functionality included in the SQL Server 2005 database engine such as stored procedures, views, triggers, cursors, CLR support, and XML support. A technical preview of SQL Server Express Edition is available for free download today from: SQL Express