The Core is a total conversion single-player mod for Half-Life. Survive the disaster as Maurice Andrews, a low level engineer stationed in Black Mesa during the events of the resonance cascade. Seemingly trapped, Andrews must fight his way to the surface to survive.
A neat look at the design process of Cendark3 - one of my maps for The Core. Note that this video begins when the map is around 2 days into development already, then continues through three days of additions and tweaks.
I really like this method of showing changes, and I'll definitely be doing it on future maps!