SQL Server Everywhere

Check out this SQL Server 2005 Update from Paul Flessner on a new member to the SQL Server 2005 family - SQL Server Everywhere. ;) Similar to SQL Server Express edition, it has a limitation of 4GB data files.

"a light weight database designed for client applications that run from desktops to mobile devices"

Check out some of the blogs too
http://blogs.msdn.com/smartclientdata/archive/
2005/07/15/439008.aspx

http://blogs.msdn.com/stevelasker/archive/
2006/04/10/SqlEverywhereInfo.aspx

http://blogs.msdn.com/rogerwolterblog/archive/
2006/04/09/572101.aspx