The Sony store has a few free ebooks. Open the Store page and on the left side of the screen there are a few links, one of which is Bargain Books. Open that and the 6 or so free ones are listed first. You can download to your computer and then install to your reader if you don't want to use the wireless. And I haven't had my reader long enough to have been watching the store to see if they offer different free books time to time.
There are a couple of Barnes and Noble owned sites that have free books. eReader.com
. However, both sites offer the same freebies.