SQL Server 2008 Express Edition is a free, easy-to-use, lightweight, and embeddable version of SQL Server 2008. Free to download, free to redistribute, free to embed, and easy for new developers to use immediately.

SQL Server 2008 Express Edition has the following benefits:

Fast to download and install: It's a compact download, and you can get up and running in as little as 20 minutes.
Simplified administration: Automatically tune your database for optimal performance. For convenience, you can use Microsoft Update to automatically receive the latest patches and updates, enabling a high level of security and the latest features.
Free management tool: SQL Server Management Studio: You can download the SQL Server Management Studio Express, an easy and efficient management tool for SQL Server Express. With SQL Server Management Studio you enjoy a consistent management experience across all SQL Server 2008 Editions.
Seamless integration with Visual Studio Express Editions.
Easy to deploy: Easily move or copy your SQL Server Express databases to another computer or hosted server using XCopy and ClickOnce deployment.