Collaborative Document Editing?

print-spool asks: "I know the feature has existed in certain products for some time, but is there a Free Software alternative for the collaborative editing of a document out there? This seems to me like a gap in the wide range of existing software, and would be particularly helpful for the improvement of HOWTOs, FAQs etc. As well as Open Books and the like. So do any of you know of such a utility, or if you are a developer do you know if such a system would be feasible to implement? " As long as the document is text (or even TeX) then CVS would work well enough. However, if the document is of some binary form then that may not be a good choice. Any other ideas?