Red Mars by Kim Stanley Robinson is being given away in a couple of formats PDF from Suvudu and Kindle. This is one of the best hard science-fiction books that I’ve read in a long, long time. In fact, I just started listening to the audiobook version a couple weeks ago, before I found out that the book was available for free, because it’s a great story.
Red Mars starts in 2026 with the first colonial voyage to Mars aboard the largest interplanetary spacecraft ever, the Ares, with a crew of the “First Hundred” colonists, composed for the most part of Russians and Americans. The book details the construction of the first settlement on Mars, called “Underhill”. A debate among the colonists breaks out about the advisability of terraforming the planet, focusing on the two extreme views personified by Saxifrage “Sax” Russell who believes their very presence on the planet means some level of terraforming has already begun and it should be continued, a viewpoint held by “the Greens.” Ann Clayborne represents “the Reds” viewpoint that mankind does not have the right to change entire planets at their will and Mars should be left in its original state. Hiroko Ai represents a middle ground, believing that a new way of living could evolve on Mars, a philosophy referred to as “Areophany”.
-from Wikipedia’s entry on Red Mars
There are a handful of other free novels as well. You can see the full list of free books on Suvudu, which links to the various formats available.