The UI was implemented using WPF4 and Blend. We also use a framework called Caliburn Micro as well as a framework I wrote called ReactiveUI which orchestrates connecting the UI to data via binding.
It looks great and I am installing it now on Windows 8. I'm curious why you chose to use Caliburn Micro over, say, MVVMLight? Do you plan to share some of the technical decisions made for this product on GitHub's Blog?
Edit for spelling
We used Caliburn Micro for its Screens / Conductors implementation and its convention-based wireup of views. We use ReactiveUI for all the ViewModel code though.
Any chance you will open source this app - it would be nice to have a real world example of using Caliburn Micro and ReactiveUI.
Thanks for the info. It looks like you chose to embed Chromium for rendering markdown? What was the reason for not using an embedded IE instance (which is already part of every Windows installation).
IE doesn't participate in the WPF rendering system, so it would have all kinds of glitches like it sitting above all other controls (referred to in the docs as "Airspace Issues"), as well as unthemed controls (specifically the scroll bar - stuck out like a sore thumb). The markup that READMEs use is really simple, IE would've been fine for it.
Airspace issues have been the bane of my existence until WPF3.5sp1. D3DImage! Side note -- I also advocate Caliburn.Micro, it's great!