Lugaru from Wolfire Games
There's no dates listed for this, just says for the holidays, so could end at any time.
Lugaru (pronounced Loo-GAH-roo) is a third-person action title available for Mac OS, Windows, and Linux. The main character, Turner, is an anthropomorphic rebel bunny rabbit with impressive combat skills. In his quest to find those responsible for slaughtering his village, he uncovers a far-reaching conspiracy involving the corrupt leaders of the rabbit republic and the starving wolves from a nearby den. Turner takes it upon himself to fight against their plot and save his fellow rabbits from slavery.
Information on the game here: http://www.wolfire.com/lugaru
The form to fill out with name and email address to have a serial number sent to you here: http://www.wolfire.com/free-lugaru
Venture Arctic from Pocketwatch Games
This game is free until New Year's.
Here is the post on the game site's forums: http://www.pocketwatchgames.com/forum/viewtopic.php?f=14&t=854
The post gives instructions, including the coupon code to get the game for free. The registration code is then mailed to you. It arrives quickly.
Information on the game itself and the download are on this page: http://www.pocketwatchgames.com/venture-arctic
A short synopsis is that Venture Arctic is a simulation game that takes place in arctic climates. You have to balance the ecosystem, ensuring there are enough predators and prey to keep things going.