Every Friday, Barnes and Noble offers a free ebook download through their Unbound: NOOK and BN eReader Blog for a limited time. They don't specify what "a limited time" means, but I was able to find books that were still free several weeks after they were originally posted.
There are some drawbacks to this. Like Amazon, they are trying to sell you their ebook reader, the NOOK. The ebooks files come in the epub format used by the NOOK. They offer free eReader software, though, that will let you open and read the file. Once you've gotten the book, just go to your My eBooks Library to download it. Once you've gotten the book downloaded you can use it on any computer or mobile device, through eReader. (You will also have to give them your credit card number, even though the books are free.)
If you're willing to jump through the hoops it's another option for free ebooks and not a bad one, either. They've offered some pretty good books this way. Just look for the posts titled "Free Fridays!"