Wiki - a term meaning "quick" in Hawaiian - is one of the long-standing online collaborative tools. Many versions are available - hosted, proprietary, free, etc. But, one feature is consistent, they all enable one to see the prior versions of the document.
Recall when you have assigned a group project. Have you ever had students suggest that "I did all of the work" - or "Ray didn't do anything to contribute to the project!" A wiki enables you to see exactly what each student contributed to the final report.