We haven't seen a new Half-Life 2 game since Episode Two came out in October 2007. According to the folks at ValveTime and Rock, Paper, Shotgun, that's partly because Valve canned Episode Four, a spooky sequel that would have taken players back to the zombie-infested town of Ravenholm. And you'll never guess who was developing it: Arkane Studios, the same folks who released the excellent Dishonored last year.
ValveTime has posted screenshots and a video of the never-released Episode Four, and Rock, Paper, Shotgun has confirmed that this is no hoax. Valve really had a third-party developer working on a Half-Life 2 episode—one whose plot was shoehorned somewhere before the end of Episode Two, so as not to disrupt Valve's storytelling mojo.
That's actually one of the reasons the episode was canned, according to Marc Laidlaw. The Valve writer and designer told ValveTime:
"As I recall, we felt like a lot of the staples of Ravenholm–headcrabs and zombies!–were pretty much played out, and the fact that it would have to take place sometime before the end of Episode 2 (so as not to advance beyond where Valve had pushed the story) was a creative constraint that would hamper the project... and Arkane."
ValveTime notes that Episode Four was in development "between 2006 and 2007," so I guess this counts as ancient history by now. Still, it's interesting to know Valve almost farmed out a bona-fide Half-Life 2 episode to another developer—and we almost got some fresh Gordon Freeman and Gravity Gun action to tide us over until Episode Three.
Speaking of which, what is happening with Episode Three?