Hibernate is a powerful, high performance object/relational persistence and query service for Java. NHibernate is a version of Hibernate for .NET.
More about Hibernate: Hibernate FAQs.
Hibernate lets you develop persistent objects following common Java idiom, including composition, association, inheritance, polymorphism, and the Java collections framework.
Hibernate is supported by JBoss/Redhat and SourceLabs. Hibernate is part of SourceLabs SASH distribution.
Hibernate is covered in SourceLabs Self-support Suite for Linux and Open Source Java
Hibernate code churn statistics are available here