You can also go to sites like manybooks.net or epubbooks.com for a whole range of great freebies - I think more than half of what's on my reader now are from these sites.
The Sony Reader store offers free books, but it's a little more complicated to find them - you have to go to the category you want and arrange them by price, then hope that there's something you want. And sadly, a lot of Sony's freebies are from sites like Smashwords and Wattpad - the former isn't so bad, but the latter is glutted with stuff written by semi-literate 14-year-olds and titled things like "I'm The Only Girl At An All-Boy's Boarding School...Oh Yeah And They're All Lust-Filled Vampires". I wish I were making that up.
