I finally got the power adapter replacement for my PowerBook, and set up proper networking and set up the firewall to allow SubEthaEdit and Rendezvous. It works great! Not only can a programmer "announce" a shared document and share it over the local network, but it can be shared over the Internet as well! Each contributor chooses his color, and everyone else connected sees any changes he's made and can also see where his cursor is and what he has highlighted at the moment.
People are using this for collaberative note-taking at conferences, paired or "extreme" programming, document editing, and anything else you can imagine. It's free for non-commercial use, and not at all expensive for commercial licenses. ($35).
I've even used it as a chat system, since whatever everyone writes appears as it's being written on everyone else's copy.
If you are a remote editor, you can save the document to your own hard drive, but the main document is the responsibility of whoever shared it in the first place; remote users won't be asked about saving if they close the document or quite SubEthaEdit. You can also share read-only, and there are a lot more options that I haven't even started to learn about yet.
If there's anybody here developing mods and snippets and such, or working up documentation, on a Mac, maybe we could get together and play around with this.
http://www.codingmonkeys.de/subethaedit/collaborate.html