Half-Life 2: Episode 1
Holy crap. $17 well spent. It's shorter than Sin Episodes: Emergence but it's a different game and the atmosphere alone was worth the price. It picks up immediately after the ending of Half-Life 2 which I won't spoil but I can safely say it kind of feels like you're working backward through the end of Half-Life 2 through the first part of the game. There's a couple new enemies, no new guns, but they do some neat stuff with what you've got such as one section where you've got little to no ammo and you're relying on using your flashlight to point out zombies in the dark so Alyx can shoot them. Valve also did an awesome job of keeping the gameplay varied. At some points you're sneaking around and conserving ammo and others you're running and gunning and later you're not using weapons at all, just the gravity gun and whatever is laying around you. The new (well, not exactly new but in greater use) HDR effects really add to the atmosphere as well. Sometimes the lights are so bright they're blinding and everything looks really good under it. It also has a "commentary track" where while you play through the game you find little blurbs that will tell about the design process of Episode 1 and whatnot, which I haven't gone through yet as I just finished the game for the first time but I'll definitely be checking out on the second play through.
Now I'm stuck waiting for winter for Episode 2.