VistaDB has an express version which is free to use and is syntax and driver compatible with SQL Server. VistaDB is a single file and only requires their driver .dll to work in your asp.net or winforms project.
Since it is syntax and datasource compatible you can upgrade to SQL Server if needed.
from their site:
VistaDB is a fully managed and
typesafe ASP.NET and WinForms
applications using C#, VB.NET and
other CLR-compliant languages.
VistaDB.net
