The DiffPlex project is a combination of a .NET Diffing Library with a Silverlight and HTML diff viewer. It is released open source under the MS-PL license.
Microsoft an ASP.Net MVC "Sample" (or is it a starter kit?) source code that can be used by Web developers to build blog engines into sites ... it's called "Oxite" and it's the software running the blogs on MIX Online. http://www.codeplex.com/oxite