Last week, for a brief period, the Humble Store gave free copies of Lego: The Hobbit to everyone who signed up for the Humble newsletter. This week, it's doing the same for Lego: Lord of the Rings, which is free until 10 am PT/1 pm ET on December 22, or while supplies last.
"While supplies last" seems like an odd caveat for a digital store, but it might be a relevant concern, as several people reported in the comments on last week's freebie that the giveaway had ended before the deadline expired. No reason was provided, but the assumption is that Humble simply ran out of codes. I've emailed to ask, and will update if I receive a reply.
You'll have to subscribe to the Humble mailing list to take advantage of the offer, but that seems like a reasonable request. If you're already subscribed, then you can just click the "get the game (opens in new tab)" button and have at it. Do note that the free Steam keys will expire on 10 am PT on December 27, so redeem it when you get it or you might miss out.
Update: Humble Bundle has confirmed that last week's Lego: The Hobbit giveaway ended early because it ran out of keys. "This was actually one of our quickest giveaways ever," a rep explained. "We ran out of keys before the end date and we hope that everyone who missed out on a key can grab a copy of Lego: Lord of the Rings (again while supplies last and we anticipate it will be just as popular!)"