Amazon's free Kindle books selection
Amazon offers thousands of books for free. This includes those with expired copyrights as well as the occasional book by a new author looking for exposure.
Check it out here >
OpenLibrary
OpenLibrary operates as a conventional library does, letting you borrow and return e-books. All it takes is a free account.
Check it out here >
Project Gutenberg
Named after the man who's credited with inventing the printing press, Project Gutenberg is filled with free digital versions of of books whose copyrights have expired.
Check it out here >
ManyBooks
An eclectic collection of free e-books, ranging from classic (The Wonderful Wizard of Oz) to the obscure (And The Kaiser Abdicates).
Check it out here >
Free Books Hub
This site offers lots of books by contemporary but relatively unknown authors for free and cheap.
Check it out here >
DigiLibraries
Books from all kinds of categories live here. Even comic books. And they're all free.
Check it out here >
Kindle Lending Library
Amazon has facilitated an e-book lending library of its own. The only catch is that you have to own a Kindle device and be a Prime member to use it.
Check it out here >
iBooks
If you browse around the iBookstore in iTunes, you'll notice that Apple is happy to make plenty of books, new and old alike, readily available at no cost. There's even a dedicated section of the store for them.