SQL Server 2008 R2

A free and feature-rich database for developing and deploying
desktop, web, and small server applications.

Easy to get. Free to use.

• Available free for both development and deployment.
• Quickly build and deploy your database solution.
• Backup and restore with ease.
• Available with a graphical management tool, reporting capabilities, full-text search, and spatial support.
• Ideal for building Web sites and apps, small scale LOB solutions or for redistribution with ISV applications.

Powerful

• 10 GB database storage.
• The same advanced database engine as other versions of SQL Server.
• Compatible with all editions of SQL Server^® 2008: Scales with your growing business needs.
• Compatible with the scalable, self-managed and multi-tenant cloud SQL Azure Database service.

Integrated
• Download the SQL Server Driver for PHP or check out the Community Preview version 2.0 which supports PHP Data Objects (PDO).
• Designed to work with Visual Studio and ASP.NET.
• Download the free PowerPivot for Excel and create amazing analysis and reports in Excel 2010.
• Synchronize with other SQL Server Databases using the Microsoft^® Sync Framework.

* Microsoft SQL Server 2008 R2 Express supports 10 GB of storage per database.

Source : http://www.microsoft.com/express/Database/