Draftback is a Chrome extension that allows you to see what edits and revisions you've made while working in Google Docs. I found it via author Jeff Somer's hilarious post about it here (and personally I agree with everything he said about the idea of recording yourself editing a doc for vanity purposes.)
The reason I'm still posting about it is for the benefit of collaborating writers who work online together. Using this could save tons of time for writing partners who can use it to see what changes have been made by the other partner versus telling them via lengthy e-mails, writing up change notes, etc. I think this might also prove helpful to indie writers who want to work online with their editors. This could also be a pretty decent teaching tool for editing, particularly with students who can follow along and see how the process works.