• http://aaaaarg.fail
• http://gen.lib.rus.ec
• http://monoskop.org
• http://scihub.org
• http://ebookcollective.tumblr.com
• http://www.freebookspot.es
• http://ebookee.org
• https://onebigtorrent.org
• http://textbooknova.com
• http://en.bookfi.org
• http://www.gutenberg.org
• http://oll.libertyfund.org
• http://theaudiobookbay.me (audiobooks)
• http://academictorrents.com/ (a site with both papers and data sets)

Not yet tested:

• http://www.manybooks.net
• http://www.feedurbrain.com
• http://www.alleng.ru/
• http://ishare.iask.sina.com.cn/
• http://2020ok.com/
• http://www.freeetextbooks.com
• http://www.downeu.me/ebook
• http://forums.mvgroup.org

A custom eBook search engine created by ManWithoutModem on Reddit*.

To convert eBooks to other formats, Calibre is free and fairly easy to use.

* Info for this post was collected from colleagues and this Reddit thread.