There's something cool going on there for just the next few days. And if you've bought an Amazon Kindle or a Sony Reader--or just like to read e-books on your laptop, cell phone, or other system--you'll want to scoot right over to the "Freebies Bonanza" page. [Update-- this content is no longer available.]
There you'll find 24 free e-books and a collection of downloadable, high-resolution cover art suitable for use as a computer desktop background.
This is all good stuff from major authors and artists. I already owned two of these books myself.
This content will only be available through Sunday. So don't delay.
And you'll also want to bookmark Tor.com, where you can participate in discussions with Tor and Macmillan employees, authors, and other science-fiction and fantasy enthusiasts. Plus, there are other free stories there at the moment, and more to come. The site is still officially in beta, so even Tor is probably not entirely sure what it will be offering over time.
Tor and its authors are following a path blazed most notably by Baen Books. The Baen Free Library has been giving away free e-books for years, attracting considerable attention in the publishing industry and--more importantly--lots of extra business for Baen's participating authors.
I don't foresee a day when free e-books will be the principal means of distribution for new titles; authors still have to make a living somehow. But as a means of attracting new readers to an author's work, offering free e-books of older releases has proven to be very effective.
Even if you haven't read a lot of science fiction or fantasy lately (or ever), try downloading some of these titles from Tor and Baen and give 'em a look.