Gobby is a dedicated editor for allowing multiple users to edit a set of text-documents (like source-code, etc.) simultaniously. Everyone sees what the others are doing in the current document.
CoWord promises the same but uses Microsoft Word as underlaying editor. This basically means a group of users can edit a document simultaniously.
Up to now I have not had time to try any of these tools, but I am planing to do so soon. Of course there are lots more tools for collaborative editing. A very comprehensive list of collaborative editors is available on Wikpedia.