There's free and then there's Free. The Creative Commons folks have just unleashed a new license that's about as free as it gets: Creative Commons 0 1.0.
Essentially CC0 allows the user to put the work into the public domain:
The person who associated a work with this document has dedicated this work to the Commons by waiving all of his or her rights to the work under copyright law and all related or neighboring legal rights he or she had in the work, to the extent allowable by law.
What's the idea here, and why does CC need a license just to put something under the public domain? As noted in the FAQ -- sometimes an author has rights attached to a work by doing nothing at all. Without explicitly releasing something into the public domain, the author retains rights that they may not wish to exercise.
So if your project should be 100% free, think about using CC0.