This is what was formerly known as SQL Server Everywhere.  If you need a lightweight database for your Smart Client applications, then you should definitely check out SQLCE.  One of the great things about SQLCE is that you can deploy it as a private assembly.  This means no admin privs to deploy a local database using ClickOnce!

http://blogs.msdn.com/stevelasker/archive/2006/11/07/sql-server-compact-edition-rc1-released.aspx

-Marc