We have attorneys on staff and looked into other tools, but understand that about half of the contracts that live on the web are not webpages, but versioned PDFs and could not find anyone that can catch when that occurs, so we built it ourselves.
When a change occurs we email a redline (diff) and in the process of working on a dashboard that you can see not only the version of the contract when you submitted it and the new one, but overtime be able to scroll through all previous versions of that contract too...very much inspired by Google Maps Street View timeline tool. :)