All books from
www.baen.com or
www.stevejordanbooks.com or
www.feedbooks.com are DRM free. As far as I know, all *.prc are DRM free but *.mobi can be protected or unprotected. Also *.azw files can be protected or not. I have found that the zero cost public domain books in the Kindle Store are generally DRM free. There are many more DRM free books than those with protection. Also, for us that cannot run Windows, I use both Stanza and Calibre. Try to open a protected book in either one and it rapidly gives you a warning that it is locked. In Calibre they can be listed but not opened if they have DRM. I had lots of books with the title but not the author and used either program to find the author and change the file name accordingly. I have over 800 e-books in my Calibre file and tag those with DRM as I find them.