Microsoft SQL Server 2005 Compact Edition

Overview (from microsoft.com)

Microsoft SQL Server 2005 Compact Edition offers essential relational database functionality in a compact footprint ideal for embedding in mobile and desktop applications including a new generation of occasionally connected dynamic applications.
Sharing a familiar SQL syntax and common ADO.NET programming model with other SQL Server Editions, and management via SQL Server 2005 Management Studio, SQL Server 2005 Compact Edition allows developers and administrators to apply their existing skills and be immediately productive.

In addition, SQL Server 2005 Compact Edition enables scale-up of an application, either via a robust synchronization with other SQL Server Editions, or by moving to a higher edition of SQL Server. SQL Server 2005 Compact Edition allows you to extend your integrated data management platform deeper into your enterprise and into your mobile workforce. Download SQL Server Compact Edition Release Candidate 1 (RC1).