This will all be linked heavily on MSDN starting tomorrow, but I got the ok to forward the already-live links tonight. :-)
In addition to 120+ pages of good word-content, almost all of the below sections also provide working C# sample implementations of the various provider types (usually going against an XML or text file for persistance).
Introduction to the Provider Model Membership Providers Role Providers Site Map Providers Session State Providers Profile Providers Web Event Providers Web Parts Personalization Providers Custom Provider-Based Services Hands-on Custom Providers: The Contoso Times One of the things that has been nice to see on the ASP.NET 2.0 forums has been how developers using ASP.NET 2.0 have really-taken to the whole concept of providers, and are evangalizing the model of building custom ones for advanced scenarios. I'm hoping that we'll shortly start to see the next wave of this soon -- where we'll see both commercial and open-source implementations start to appear of custom providers that others outside Microsoft build.
The beauty of the provider model is that all of the higher-level features in ASP.NET 2.0 that build on top of them (Login Controls, WebParts, Treeview/Menu, Compilation, Profiles, Admin Tools, Heath Monitoring, Roles, etc, etc) will just work with these new custom implementations -- which should hopefully blend the best of productivity with advanced customization.