Tor Books is giving away free ebooks and wallpapers in an effort to promote their new site. The promotion lasts until the new site launches. It looks like the new site site may be some sort of social network for SF&F fans.
Found a very interesting article on the effects of Tor's giveaway on book sales. Bottom line: some authors see no change, others experience a big bump. Especially if they give away the first book in a series, then sales for subsequent installments increase.
The website is Tor.com.