From Microsoft:Microsoft SQL Server 2008 Express teaches you to build desktop and small server applications and redistribute by ISVs.
Microsoft SQL Server 2008 Express (64-bit) 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.