Books can be quite expensive. Sometimes, especially when doing research, it's also great to be able to search through the book easily, like you can do with an ebook. This list is partly done for selfish reasons, so that I can easily find these sites again for myself. But for you, my friend, here is a list of 6 great places to find ebooks for free.
Bartleby - providing students, researchers and the intellectually curious with unlimited access to books and information free of charge on the web.
The Internet Public Library - Huge selection of more than 20,000 books to read online.
Many Books - Over 21,000 free ebooks available here.
Project Gutenberg - Over 25,000 free ebooks in their online catalog.
Refdesk - Quality, credible and timely online resources available for free, such as dictionaries, encyclopedias, calculators and much more.
The Online Books Page - Over 30,000 free books online, hosted by the University of Pennsylvania.
This list is compiled from Here where 40 sites are listed. But most of the links either didn't offer much or didn't seem quite as free as they appeared at first glance, so I've pared it down.
There are many other resources out there. But these seem like the best of the bunch. If you find more places to get great books online for free, leave a comment.