I prefer to use sources that require attribution. I think it is important to be able to trace the ideas back to their creators. I want my students to realize that information doesn't grow on trees or magically appear, people create it. By inference this means that they, too can create information. I don't need it to be fully in the public domain to be useful, but it does work better most times for my purposes if derivatives are allowed. I am all about the mashup. I like my students to put together and connect things from different sources, showing that they get the original significance and know where it came from (attribution). If you can't make derivatives, it becomes a static piece, which seems to me to be much less useful to both student learning and my teaching.