MonoRail is a Model-View-Controller framework that can be used as an alternative to standard ASP.NET WebForms. It runs on .NET 1.1/2.0 and Mono. It enforces the separation between model/data, controller and the view. There is a Getting Started guide that has a pretty comprehensive walk-through of a rails app, including using the built-in support for the Active Record pattern to persist objects.
The Castle Project itself is a collection of projects for .NET web development.