You can find a free download of a new version of Trans-Siberian Orchestra's track "Dream Child" available on Amazon right now. This one is much more of a melodic ballad then the rock/orchestral mash-ups that the Trans-Siberian Orchestra is generally known for, but it is a very pleasant, dreamy piece.
If you like instrumental electronic rock Christmas music in general, you might want to check out Mannheim Steamroller -- unfortunately not free, but definitely my favorite for that style.
There's a veritable bonanza of free holiday music available this Christmas season! Take a peek at the rest of my findings so far, and check back every day for more!