In writing multiple versions of articles I need to have a way of measuring the degree of uniqueness the different versions have from one another. Are there any programs or websites available (preferably free ) that can take two versions of a text and comprehensively analyze them for uniqueness/similarity. Obviously the best tool would see uniqueness/similarity in the way that search engines measure them. Any consistent method would be helpful for ensuring that my different versions won't be seen as duplicate content.
Copyscape is used by most people here. I don;t know any tool that does it for free. Copyscape is pretty cheap though.