There is a ton of free stuff available, although often for a limited time. I subscribe ($0.99 per month) to a daily blog (Free Kindle Books, by Michael Gallagher) from Amazon that lists the new books available for free that day. While the biggest category would be business books, SF would probably be the second. I got Charlie Bamforth's "Beer is Proof..." book free this way. I've probably downloaded nearly 100 books in the six months I subscribed to this blog.
Another source is Phoenix Publishing, http://www.phoenixpick.com/catalogue/PPickings.htm
. They have a lot of great authors in their lineup. Once a month they make a title available for free, though you have to subscribe to their mail list to get it (they send out one message a month - to tell you what's free!). This month it's Radix
, by A. A. Attanasio. The code to get it is 9991421.
Other sources include:
, everything is free.
Project Gutenberg, http://www.gutenberg.org/catalog/
, for older stuff