Valve is probably working on Half-Life 3, but until we get an announcement, all we can do is dream. But there are things you can do to make the time go by as a Half-Life fan, such as checking out the Unreal Engine 4-powered Half-Life 2 environment project by Polycount member 'PogoP'.
The screenshots alone do it justice, with PogoP explaining: "Just so you know, I do plan to release this scene online for people to play around with. I'm not going to release it just yet, as I would really like to get it working with Oculus Rift before I do.. So you can run around with virtual reality. Not entirely sure if its going to work, but I'll give it a good go!"