Sketchup Source Tools -- that is, Sketchup --> Half Life, Portal, Counter Strike, Day of Defeat, and Left 4 Dead. Both as models (which has been around for a while) and as rudimentary level design. Woot!
In other news, I grabbed L4D from Stream while it was on sale for $7.49 ... yeah, I'm behind the times, but I'm gamer-cheap :)