SQL Server Everywhere is a new, lightweight database you can load within the process of your application. We first announced this new SKU a few months ago, and it's an exciting addition to the SQL Server platform, providing a great solution for apps that use and keep data, but don't need a "server" per se.
If you're a data application developer, this is a release you should familiarize yourself with.
Details: SQL Server Everywhere CTP Books Online
Product site : http://www.microsoft.com/sql/ctp_sqleverywhere.mspx
Steve Lasker's post is also worth a look: http://blogs.msdn.com/stevelasker/archive/2006/06/12/SqlEvCTP.aspx