calibre is compiling a list of drm free books from various sources. I am doing this in my free time so its going to proceed slowly but we have got a start on it. Once we have a sizeable number of e-books on our list (few 100) we will link to it from the calibre website or embed it there.
Anyway for those of you who write/publish DRM free books, we would like to include you on our list. Please pm if you are interested.
The website with about 50 books on it has been set up and is available at
http://drmfree.calibre-ebook.com/