List of possible tools
DocSynch - Plugin for JEdit, tests by Arje Cahn have shown that it requires exclusive locking of the document.
Reduce - currently they provide a single "demo" account, trying to edit the same document from two different systems crashed the app, but this might be due to the same username being used twice. I have contacted them to find out -- Bertrand Delacretaz.
(There was a table here, meant to compare tools but it was empty. If you need it you can get it back from version 1 of this page).