You should check out OpenRasta. It's a resource-centric framework explicitly designed for implementing RESTful architectures in .NET, and features strong support for things like HTTP content negotiation and digest authentication.
OpenRasta's approach is that instead of implementing your API in terms of verbs (actions), your API should be defined in terms of resources (which will typically map closely onto your API entity model) and codecs, which provide decoupled serialization/representation of those resources as XML, JSON, HTML or any other content format your API needs to support.
It's open source, written entirely in .NET, includes built-in support for IoC and dependency injection (that's how lots of it's wired together internally), and distributed under the MIT license.
Version 2.0 has been stable for a while now, and is being used in production at several places - most notably Huddle.
In my opinion, OpenRasta's strong focus on resources means it's much closer to Roy Fielding's original vision for RESTful architecture than many of the "multi-purpose" web/HTTP frameworks, including WCF and ASP.NET MVC.
