In a recent Reddit AMA with Valve head honcho Gabe Newell, among the sea of Half-Life 3 questions, the somewhat more sensible "Is Valve still working on any fully-fledged single player games?" was asked
. Cue the audible from the International Space Station sound of readers adjusting their posture to the more ergonomically friendly upright position as they awaited the response.
And the answer from Gabe was simple one word, "Yes." Now, make of that what you will. It's neither here nor there, and could very well be in reference to Valve's unique setup where developers are free to experiment and work on projects that may never see the light of day. But, Gabe also re-iterated Valve's hope that the Source engine would be adopted en-masse for game development. And that internally Source was the foundation of a few unannounced products.