Earlier than that; I wanted to say EditPad but that's not it. I found it; EtherPad in 2008 is the one I was thinking of, but the history of collaborative editing apparently goes back to Doug Englebart's Mother of All Demos in 1968:

https://en.wikipedia.org/wiki/Etherpad

https://en.wikipedia.org/wiki/Collaborative_real-time_editor...