calibre recently introduced Open Books, a site for easy browsing of DRM-free e-books (e-books without DRM) that are not in the public domain.
From their website: calibre has a cornucopia of features including library management, e-book conversion, syncing with devices, news download, e-book viewing etc, but to make the most of these features with your e-books you need to ensure your e-books do not carry DRM. Open Books is a compilation non DRM e-books from various sources linked to enable readers to browse and download them. Open Books invites you to submit links to DRM free e-books that you would like added to the database. Open Books invites you to submit links to DRM free e-books that you would like added to the database. Contact us with details of your DRM-free e-books on the calibre facebook fanpage or #calibreforum page. Please do not include links to books on Smashwords or Baen as we are already aware of them.