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.
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.
It is 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 also designed to integrate seamlessly with your other server infrastructure investments.
You can download Microsoft SQL Server 2008 R2 Express Edition with Service Pack 1, here.