NHibernate

NHibernate is an Object-relational mapping (ORM) solution for the Microsoft .NET platform: it provides a framework for mapping an object-oriented domain model to a traditional relational database. Its purpose is to relieve the developer from a significant portion of relational data persistence-related programming tasks.
NHibernate is free as open source software that is distributed under the GNU Lesser General Public License.
NHibernate is a port of the popular Java O/R mapper Hibernate to .NET.
A partial implementation of a LINQ provider is available for 2.1. A new LINQ provider using the AST parser built for NHibernate's HQL (Hibernate Query Language) engine is in development for version 3.0 of NHibernate. It is currently available for preview in NHibernate 3.0.0.Alpha2 (at 2010-08-15)