Operation Flashpoint / ARMA: Cold War Assault. When you play the games during Christmas many trees are replaced with Christmas trees with gifts lying next to them (well, actually I only checked with the original OFP but I presume that it's still the case in ARMA: CWA). I haven't ever checked whether they did the same thing in the later ARMA games.

Also Unreal Tournament 2004 had an amazing Christmas special, I think it was online only and don't know whether they are still doing it (and am not completely sure whether that was something done by devs or the community but it happened on every single server I played the game on). They would replace Capture the Flag with Capture the Tree, obviously the flag would be replaced with a tree, characters would all wear Santa hats and I think sound cues from the original Grinch cartoon would play.

Dungeon Defenders has Silent Night and Winter Wonderland.

Oh God, don't get me started on that game. One of the reasons why I couldn't get into it was because when I gave it my first try several months after Christmas there was still Christmas stuff everywhere.

Wolfenstein 3D Xmas Edition

The Amiga game Fire & Ice had a free Xmas edition, similar to Xmas Lemmings. And the second James Pond game, Robocod, is all Xmas based.

BIT.TRIP RUNNER has Commander Video with a Christmas hat during December. At least it does on PC, haven't tried it on Wii yet.

Doesn't Dungeon Keeper 2 have a Christmas theme when you play on that day? Or was it just an announcement-type deal?

Hah, never mind. Should've reread a little closer.

SPAZ has a holiday mode that allows you to shoot santa and pinch his presents, or something to that effect.

Star trek online and World of Tanks both have christmas themes in the game.

Age of Empires III has their "Winter Wonderland" add-on.

http://www.ensemblestudios.com/all_downloads.aspx (scroll down a bit...)

Cool! That one I might actually look into. :D Thanks.

Let me know how it works...! I downloaded it, but I haven't actually tried it yet. :(

It won't be soon, I need to reinstall AOE 3 first and I'm kind of backlogged. I've got the site bookmarked though.

A common theme around here, it seems. *sigh*

My backlog is huge also, and I'm wondering what games GOG will force me to buy during the winter sale, thus increasing my backlog.

Droid Assault (as seen in the current Humble Weekly Sale) has an Xmas Mode which is rather cute. And the looping holiday songs can drive you mad, just like in real life.