In my books, CC By is the only free license.
[From Whenâ€™s an OER not an OER? Â« Learn Online]
In my books…. CC is just another copyright regime. Public domain is the only free content. If you don’t put it into the public domain, all you are saying is ‘restricted’. CC just says, under the terms of copyright, you can do x. Public Domain says, you can do x and whatever else you want. I also like artistic license of Perl, though it is a copyright license.
on another note, the post above has some really great content linked from it.