Smashwords is an ebook retailer and distributor. Authors and small press publishers use Smashwords to sell their books on that site, and to push them out to retail partners, like iBooks, B&N Nook store, Kobo, and others. The summer sale lasts through the end of July and applies only to sales directly from Smashwords. However, since they provide ebooks in both epub and Kindle formats, it's not limiting to buy from them. Their FAQ has lots of info on how to get the ebook file to your Kindle or other ereader.
All my books on Smashwords (which is all my titles except for Turnabout) are either free or half price ($1.50 instead of $2.99) during this sale. Check out my Smashwords author page to see my books or browse the catalog of sale books. Use the special coupon code (e.g., SSW50 for half price books) when you check out.
Please post a comment here if you have any questions on or problems with the sale.
And have a happy 4th of July!