Yesterday, the Firebird team released their .NET provider for the Open Source database Firebird (based on the Interbase code). The provider is also open source and of course free. I haven't played with the Firebird database recently but according to the features Interbase had and looking at what the Firebird team added / updated in the source code, it is a massive alternative for those developers who need a true RDBMS with ACID compliance throughout the system and full stored procedure and trigger support but can't afford SqlServer (or alternatives which cost even more money).
The links:
Firebird works on a flavor of operating systems including windows.