NHibernate is a .NET port of Hibernate for Java. From the website:

“[NHibernate] handles persisting plain .NET objects to and from an underlying relational database.”

NHibernate has close ties to the well known and well documented Hibernate project, so there is lots of information available. This is on the must-try list, as soon as I get a spare second and a half at work.

Other O/R mappers include LLBLGen Pro and WilsonORMapper.