Textflow is an application that compares up to 8 different versions of a document.
Although still in Beta, and currently updating their desktop application, it is available in a new online service called CompareMyDocs.com.
It is not yet ready to be used for documents with tables and images and it is unfortunately Microsoft Words based, but for drafts saved as a .doc or .rtf (which almost all wordprocessors can produce) it works very well.
I actually think it might be easier to edit a rudimental version of a document in this way than by adding highlights and notes. As CompareMyDocs.com automatically does all the colouring and adding it saves a lot of mouse clicks or keyboard strokes.
Developers are welcome to sign up for a free API of the software while the rest of us can use the online service or wait for the release of the new desktop application, which will certainly be announced on the TextFlow Blog.
The company apparently values the benefits of good looking software and homepages, including their marketing products, like this pleasant flash-animation or the introducing flash video with some basic sign language ;-)
You can have a look at textflow by surfing to the testdrive.