SQL Server 2008 Express is a free edition of SQL Server that is ideal for learning and building desktop and small server applications, and for redistribution by ISVs.
Microsoft SQL Server 2008 Express is a powerful and reliable data management system that delivers a rich set of features, data protection, and performance for embedded application clients, light Web applications, and local data stores. Designed for easy deployment and rapid prototyping, SQL Server 2008 Express is available at no cost, and you are free to redistribute it with applications. It is designed to integrate seamlessly with your other server infrastructure investments. For more information about SQL Server Express, including other versons and downloadable components now available, see Microsoft SQL Server Express.