Today's mystery game is Severed Steel.
GOG has another freebie. For the next 72 hours, you can get a free copy of Worms Revolution Gold Edition.
as for the Dec 29th freebie, the leaker had this to say;
As much as any of you, we would love to know what mystery title will be available for free on 29 December. Alas, billbil-kun is not planning on spilling this many beans. The reason for that is "... it's a good franchise, and it doesn't deserve to be leaked this way.".
"I expect death to be nothingness and by removing from me all possible fears of death, I am thankful to atheism." Isaac Asimov