While we’re on the subject of top quality science fiction that you can read for free, I’ll take a moment to commend Blindsight by Peter Watts. Watts writes diamond-sharp super-hard science fiction, and Blindsight has received mountains of critical acclaim, hotly tipped to be an award winner in the coming year. He’s also made it available as a free downloadable PDF file on his website (along with his entire backlist), so go grab it and get reading – you’ll want to buy a copy once you’ve started, trust me.