Station 51 is a single player mod for Half-Life 2: Episode 2. I've been working on it for a year or two, filling my nights and weekends. I've already released the first map, which you might have seen, but I've polished up and added some bits here and there, along with a brand new second map that brings it to a satisfying conclusion. It's action heavy, with lots of classic Half-Life 2 combat, so if that's your bag, be excited.
I finally have the maps at a point where I think they're worthy of being a packaged mod, and hopefully soon I'll be releasing the whole package. I'm in the final stages of testing and polish before I release, so fingers crossed, it should be out soon!