Gutenberg.org
Gutenberg.org is one of the most famous sources for free ePub-books. They offer about 30,000 books in many different languages.
Website language: English
Manybooks.net
Manybooks.net offers more than 26,000 free books in many different languages. Definitely a place to go when you are searching for free books.
Website language: English
Mobileread.com
Mobileread.com offers more than 1,800 free ePub-books. Most of them are English but there are also many in German. The Mobileread.com community is very active, so there are added several new books almost every day.
Website language: English
Mendele.co.il
Mendele.co.il is the first website which offers ePub-books in Hebrew. As the website is pretty new, the number of offered books is not so large in the moment. They charge for the most books, but offer some for free. If you buy a book, the book is delivered with a watermark and is not protected by DRM.
Website language: Hebrew