Jeff Harris is without a doubt an awesome programmer.
What blew my mind back in the day was my discovery of his OpenC1 - an open source port for good old Carmageddon game from 1997.
He found a way to read all original Carmageddon files (how to deal with textures, sprites, animations, 3d models etc) and wrote his own engine using C# and XNA.
On his website you can read more details about OpenC1 development and other interesting things (for example episode with Square Enix, a company that currently had the rights to Carmageddon trademark)
When I have some time, I'm trying to dig into his code and implement raytracing engine for OpenC1. But saddly I'm very busy these days.
Source code on Github: