Data Storage

Databases

Oracle Express Edition
Oracle Database 10g Express Edition (Oracle Database XE) is an entry-level, small-footprint database based on the Oracle Database 10g Release 2 code base that's free to develop, deploy, and distribute; fast to download; and simple to administer.
http://www.oracle.com/technology/products/database/xe/index.html

Microsoft SQL Server Express
Free version of Microsoft's SQL Server RDBMS
http://www.microsoft.com/express/Database/

SQLite
SQLite is a software library that implements a self-contained, serverless, zero-configuration, transactional SQL database engine. SQLite is the most widely deployed SQL database engine in the world.
http://sqlite.phxsoftware.com/

ORM Tools

NHibernate
NHibernate is a mature, open source object-relational mapper for the .NET framework.
http://nhforge.org/Default.aspx

Fluent NHibernate
Fluent NHibernate offers an alternative to NHibernate's standard XML mapping files. Rather than writing XML documents, you write mappings in strongly typed C# code. This allows for easy refactoring, improved readability and more concise code.
http://fluentnhibernate.org/

ADO.Net Entity Framework
The ADO.NET Entity Framework enables developers to create data access applications by programming against a conceptual application model instead of programming directly against a relational storage schema. The goal is to decrease the amount of code and maintenance required for data-oriented applications.
http://msdn.microsoft.com/en-us/library/bb399572.aspx