This may be a hoax, I've no idea. But it's a holiday here today, so that's the right sort of day for posting suspect things. Thanks to @PaT2090 for linking us to what was allegedly briefly appearing on Ellis Savannah's Facebook page (L4D's Ellis, of course). An apparently intro screen for Half-Life 2: Episode 3. See it in full below.
It's a fake. Of course it is. It's ages old, too. But, see, all your rude-faced people calling me names - I haven't seen every image of everything on the internet, and so, you know, can be fallible. I'm sorry to break my non-omnipotence to you this way, but there it is.
Now, obviously no one doubts that Valve must be developing Episode 3, despite their never having officially announced it. And of course this could easily be a fake. But who cares?! Vicious rumours power the internet, like insane mice in a wheel made of crazy.
Click for embiggenment: