Microsoft MVC Framework.
Scott Guthrie just recently announced the ASP.NET MVC Framework. Which is promises to bring similar benefits to Ruby on Rails, but looks to be mega extensible of of course be build completely on dot.net 3.5
Great video:
https://weblogs.asp.net/scottgu/archive/2007/10/14/asp-net-mvc-framework.aspx
Man does scott know his stuff...