....SQL Server 2005 Compact Edition
Microsoft SQL Server 2005 Compact Edition (SQL Server Compact Edition) is designed for developers who need light weight, in process relational database solution for their applications that can be developed and deployed on desktop as well as on mobile devices. SQL Server Compact Edition Runtime can be used to develop and deploy applications on desktop.
There is a really good independent review to be found here
(Developer.com - January 15, 2007)
"Although SQLce is billed as a lightweight relational database engine, it is not light on features. A single article could not hope to cover the full breadth of SQLce, so I am going to examine the features of SQLce that I feel are most important:
SQLce is still in beta but is available for download.