Baen has a free eLibrary at http://www.baen.com/. It's mostly SF and Fantasy.
Also, if you do an internet search for "free Baen CD" you can download ISO or Zip files of CD's containing free eBooks that Baen used to give away in some of their hardcover editions and encouraged people to share them. There are hundreds of eBooks on them.
If you sign up for the mailing list at http://www.phoenixpick.com/ you will get a code every month for a free eBook they are highlighting.