Looks like lots of free books are available online these days.
First, there's the Baen's Free Library (tons and tons of online books, free, for your reading pleasure):
http://www.baen.com/library/And if you're looking for newer releases,
http://www.tor.com is doing a "watch the skies" promotion and giving 12 e-books away for free.
So far they've given Mistborn by Brandon Sanderson, Old Man's War by John Scalzi, Spin by Robert Charles Wilson, and The Outstretched Shadow by Mercedes Lackey and James Mallory.
Scalzi's blog
http://www.scalzi.com/whatever/ points out a few other online books:
(start snippet from his blog)
* Jeff VanderMeer and PS Publishing are jumping on the free eBook train and releasing Jeff’s upcoming book
The Situation for you to peruse and enjoy, and
you can find it here, along with an interview with Jeff and his wife (and
Weird Tales editor) Ann. Jeff say this about his book: “it’s like Dilbert-meets-Gormenghast, as Margo Lanagan put it.” Hey, Jeff, you had me at “Gormenghast.” If you end up loving
The Situation, as I’m sure you will, Jeff has put up some information on how you can procure
the signed limited hardcover editions of the book.
* Also for your free electronic reading enjoyment: Butcher Bird, by Richard Kadrey, by the good graces of Night Shade Books. The link to the download is at the bottom of the page I link you too, and is available in a variety of formats. Again, if you like it, consider buying a hardcopy. Authors like to eat, and as much as I like the idea of giving people these free online tasters, I do hope people take to heart that on the other end of these works are people who worked to bring them to you. Reward them if you like their efforts.
(end snippet from Scalzi's blog)
And, on Neil Gaiman's blog
http://journal.neilgaiman.com/ , it says that during March you can read(not sure if it's downloadable) one of his books:
For the next month, your free copy of American Gods is waiting for you at
http://browseinside.harpercollins.com/index.aspx?isbn13=9780060558123&WT.mc_id=author_AmerGods_FullAccess_022208So, go read!