As I'm sitting here thinking up categories, open source projects written in .Net comes to mind. Of course the obvious first choice is Subtext, the blogging engine behind this website.
One of the biggest things I like about Subtext is the skins. I'm not going to go into creating skins. There's plenty out there for creating Subtext skins as follows:
Skinning Subtext (Part 1)
skinning subtext (part 2)
Creating your first subtext skin Part 1
In the past, I've dug up open source css templates and applied them to the naked skin as an entirely new skin. It works like a charm. I'll probably do this again very soon.