The first week's free book is Mistborn, by rising fantasy star Brandon Sanderson. Next week's will be Old Man's War by John Scalzi, 2006's winner of the John W. Campbell Award for Best New Writer. Over the next several weeks, other books still.John Scalzi has said that the Tor books will be DRM free.
There's a similar offer over at Eos, too, as they celebrate their 10th anniversary with a free e-book every two months. The first book on their list is Robin Hobb's Shaman's Crossing. However the Eos books are most definitely encumbered with DRM...
Of course we shouldn't forget Baen's Free Library, which adds new titles every couple of months in DRM-free formats.
Spotted originally on Whatever, and since seen on various folks blogs