The NHibernate Data Layer Generation (nhib-gen) project on SourceForge (http://sourceforge.net/projects/nhib-gen) is an open source project, which includes a set of base classes and templates to implement an entire NHibernate solution.
These templates are database-driven, so your model is the database and the rest of the data layer is generated from it.
One caveat of nhib-gen is that you will need a CodeSmith license. Work is underway to implement the templates in other code generation tools such as MyGeneration.
The nhib-gen project templates are the ones that I use in my development, and they have been used by several companies for large and small projects. They combine a set of base classes designed from "Best Practice" NHibernate documents and websites with generated code to provide the best possible combination of performance and ease of use.
Once the basic configuration is done, nhib-gen will create a full set of common objects (POCOs), business objects (wrappers for POCOs where...