And don't forget
Munseys. Of course, they have lots of free public domain ebooks. They also have reader reviews and allow users to tag books, so that can make things easier to search -- and sometimes harder to search because there is so much on the site, and so many possible tags.
The site isn't what it once was. The owner hasn't blogged since last June, for example. But there is still a lot there, including a lost of post-1923 pulp fiction that fell into the public domain. And a "Browse by Tag" feature that shows the most popular tags.