I really like Creative Commons. On Flickr my images are licensed with attribution, non-commercial, share alike. This makes it easy for people to reuse the images (and they do) without asking. I set it up this way so that people don't have to ask. Some still do, but the point is, they don't have to - which is what I really like. Material is available for reuse, without it being complicated, and rights are managed in a fair way. I like this for when I want to reuse content too.