Since there's not much "Steam" stuff to talk about, I thought it would be good to mention any games that have added free Christmas content, just in case anyone wanted to, you know, play one. (**hint** click the PLAY button on your Steam library page)
1. Rocket League - added a snowy arena and holiday drops.
2. Clicker Heroes - Christmas minigame and quests
3. Hitman - holiday mission
4. Infinite Minigolf - Santa's Workshop course
5. Payday 2 - stuff
6. Last Hope Tower Defense - defend santa versus zombie hordes!
This last one, Last Hope Tower Defense, is one to which I want to call special attention. This is one of those games, like Nation Red, where the developer just keeps on adding more and more content free of charge. I like that and it makes me want to buy other stuff that they do.
What is Last Hope Tower Defense? Unsurprisingly, a Tower Defense game. Where it stands apart though is in the mashup of zombies and Native American stuff. Even some of the towers are unique, like the arrow and poison turrets. It's a hero-based game that allows you to manipulate up to 4 of them simultaneously, in addition to placing towers and collecting resources. Each turret has a branching upgrade tree and all heroes have many available upgrades, just to keep things interesting.
Now I like TD games and I also like to obsessively upgrade stuff, so this game hit all the right notes with me. Not right away, mind you, because some of the early levels were more linear than I'd like. Later levels, though, demand that you defend many points from many directions. There are even some hero-focused arenas thrown in just for fun.
Give it a try if any of that stuff interests you. There's a lot of content thanks to a dedicated developer.
Please share if you know of a game that is adding/has added special holiday content.