SubEthaEdit
www.codingmonkeys.de/subethaedit/index.html
From the page
Saved with the link. Notes from 2007 to 2016 are mostly the page's own words, so this is probably an excerpt.
SubEthaEdit is a powerful and lean text editor. And it's the only collaborative one you can actually use. By combining the ease of Bonjour with the world's best text collaboration engine, it makes working together not only possible but even fun…