Smashwords is having a sale for the month of July. I have no idea why it's called a "Summer/Winter" sale. The sale is open to authors who choose to opt in and, by default, includes all ebooks that are regularly priced as "Free," which makes the search process a bit cluttered. Authors may choose to add any of their books to the promotion at any time during the month, so a title that's not on sale now may be on sale in a week or two.
At the main page, look for the yellow box on the left side that says, "Browse Books on Sale" and click it. This will bring up the complete list of all books with coupon codes
AND all books that are regularly priced as Free. At this point you can further refine your search inside the promotion by selecting the Category/Genre on the left side and the sorting criteria (for instance "Newest" or "Highest Rated") at the top. If you are interested in seeing only those titles that have a "100% Off" coupon code, but not those items that are regularly priced as "Free" use the appropriate box at the top of the page.
http://www.smashwords.com/