I am working on some customisations of SharePoint authentication at the moment. These articles give some useful examples.
SharePoint ADAM Role Provider
http://www.sharepointsecurity.com/content-183.html
ASP.NET Provider Client Application:
http://www.sharepointsecurity.com/content-185.html
More about ADAM:
http://technet2.microsoft.com/Office/en-us/library/678d6c9e-12cb-44fb-8936-7a3a67f53d3e1033.mspx?mfr=true