Source Speedrun Tools. SST is now widely used by speedrunners of both Left 4 Dead series games and Portal 1, and over time aims to be useful in other Source games such as Half-Life 2 and its mods as well. Its development has been helped along by several cool people; go read more about it on the page!
Other random things I’ve worked on are a bit scattered around and not quite so well-documented over here, but for the curious, links to some of those things are listed here:
build (I unfortunately lack time to develop it further).
I'm also a big film fan and have a fairly long list of recommended movies for anyone interested.