Google has announced that it will make over a million books that are out of copyright available to download free of charge in the new EPUB format. With the addition of support for the EPUB format, it will make downloading these titles much more accessible to users of mobile devices such as netbooks, phones, and e-ink readers. These devices typically have smaller screens that won't display image based PDF files correctly, necessitating a different format to display them correctly. This from the Inside Google Books blog:

EPUB is a lightweight text-based digital book format that allows the text to automatically conform (or "reflow") to these smaller screens. And because EPUB is a free, open standard supported by a growing ecosystem of digital reading devices, works you download from Google Books as EPUBs won't be tied to or locked into a particular device. We'll also continue to make available these books in the popular PDF format so you can see images of the pages just as they appear in the printed book.

The XML-based, open e-book standard can be tweaked for individual devices. While the Amazon Kindle doesn't natively support it, and Barnes & Noble is working on it's own proprietary format for it's Plastic Logic reader, the Sony Reader Daily Edition works with EPUB and therefore files purchased from Powells.com, BooksOnBoard, NetGalley, or it's own eBook store from Sony. 

With the popularity of new reading devices such as the Amazon Kindle and Sony Reader Daily Edition starting to gain in momentum, this is another step forward in the eventual transition from paper based books to digital ones.