Here is the latest in my link-listing series. Also check out my ASP.NET Tips, Tricks and Tutorials page and Silverlight Tutorials page for links to popular articles I've done myself in the past.
You can also now follow me on twitter (@scottgu) where I also post links and small posts.
Custom Route Constraints in ASP.NET MVC: Keyvan Nayyeriu has a nice post that discusses how to create a custom route constraint in ASP.NET MVC (one of the extensibility points in Simone’s list above). You can use these to control whether a route rule is used or not, and they can enable some pretty rich routing scenarios. Note that in addition to creating route constraint classes, ASP.NET MVC also supports using Regular Expressions and HTTP Method filters to constrain routes as well. Keyvan is the co-author with Simone of the Beginning ASP.NET MVC Book (free chapter available).
Hope this helps,