Presumably, this is the end of an era. In the wee hours of the morning former Valve employee and Half-Life writer Marc Laidlaw published a story on GitHub which appears to be a full summary of the story planned for Half-Life 3 if it were to ever be made. The story (which can be read by clicking the hyperlink prior to this) is penned by main man Gordon Freeman and chronicles the events of the game following Episode 2 of the series.
As with all things related to Half-Life, the internet is going berzerk, with the main reason why being that many believe this is an indicator that Half-Life 3 will never happen. If it is, chances are there are a ton of Valve employees currently pissed off at Marc Laidlaw as he might very well have spoiled the story they've been developing for years now.
There's also a possibility that the game is still in the works and that whatever story is being used now is not one laid out by Laidlaw. Laidlaw might have heard this and in response, rushed to get his story out there so people could see the ending of the franchise in the way he wanted to tell it. I don't necessarily believe this theory, but the truth is we really don't know what is true and why this was released, to begin with. For that reason, we should all just wait and see what Valve's Gabe Newell has to say, if he says anything following this crazy day in gaming.