"Valve engineers and artists regularly contribute to industry and academic conferences, covering topics from the high level goals of our cabal development process to the inner workings of cutting-edge rendering algorithms.": Valve publications (all pdf, but that link is html) contains articles and slides from presentation pertaining to Valve games from the original Half-Life up to Left 4 Dead 2.
I didn’t realize this site existed until I used… Googletm. I’ve already posted The AI System of Left 4 Dead 2 and Illustrative Rendering in Team Fortress 2. Other highlights include Shading in Valve’s Source Engine (pdf, slides) and Replayable Cooperative Game Design: Left 4 Dead (pdf).