Now here's a free book I can recommend with no reservations at all -- Red Mars by Kim Stanley Robinson. I've read the entire martian trilogy, and while I was lucky to find an oversize used copy of Green Mars, I had to read Red Mars in Mass Market Paperback form which was very hard on my eyes. (I think I checked Blue Mars out from the library.) I'm so glad to have this for the Kindle because I can see myself reading it again.
Don't have a Kindle? Well, if you have and iPod you can read it using the Kindle app. I've loaded the Kindle app on my iPod touch and it seems to work quite nicely. (Although it doesn't really compare to the ease of reading on a Kindle in my opinion.)
You can read about free books and more on the Kindle Blog.