Live-brary is an excellent source for free downloads. In addition to that, you have these other options:
Inkmesh is a search engine for free eBooks. Links on the front page will list free books for Sony Reader, Nooks, Kobo, Kindles, etc. Put a title, or author, in the search box, and click on The "Find free eBooks" tab.
Project Gutenberg is an index that can be searched and browsed by author or title. It offers 36,000 free quality eBooks to download to your PC, Kindle, Android, iOS, or other device. By clicking the tab entitled Top eBooks along the search bar, you will find the most frequently downloaded eBooks. Advanced searched allows narrowing by format, e.g. epub.
Globusz offers lots of free eBooks that can be read online or downloaded in txt format. Access to eBooks is through 20 categories, through a list of authors in the center of the page, and through a list of featured authors on the left side bar.
Search-ebooks.eu is a free search engine that provides an opportunity to search for and download various (pdf,doc,ppt,rtf,xls) documents.