Half-Life 2: Aftermath leaks Episode 3 beta content
The mod includes several maps leaked from the in-development Half Life 2: Episode 3, including 11 levels which were developed around 2012 / 2013. These levels are not fully realized, and are largely gameplay experiments, rather than containing storyline elements.
Additionally there are several beta test levels to demonstrate NPC behavours, and concept weapons, such as a Combine laser gun that can blast holes in walls, which would have appeared in Episode 3.
There's some interesting new NPC characters included also, such as a heavily armoured Combine soldier, and and a City Scanner with a laser gun attached to it.
The mod is available for download, for now at least, here.