In anticipation of Scott Westerfeld's alternate history novel Leviathan, Simon & Schuster has made his book Uglies available for free download. It's the first book in his young adult series about a strictly regimented, post-scarcity society where all people undergo an operation at age 16 to make them pretty, and Tally Youngblood, a teenager who stumbles upon the sinister truth behind the operation. Of course, it's all an evil plot to get you hooked so you'll buy the rest of the series. [Simon & Schuster via Westerblog]
There's a ton of free short science fiction on the Project Gutenberg site - someone is sorting through the short stories form the early days to see which ones have expired copyright and which ones haven't, so they can post the former. Some awesome stuff there.