Christopher Allen and Kalle Alm have been hard at work creating Synchroedit -- an open source synchronous editor for the web. Socialtext sponsored this open source project because many of us have waited years for a SubEthaEdit for the web that we can hack upon. Unfortunately you have to wait a bit more, it still needs IE support and hasn't been integrated with Kwiki and Socialtext, but you can go play with it now.
The developer wiki is here.
UPDATE: Chris Allen, in comments:
One key point that people seem to be missing is that SynchroEdit is an open source project. This means that we are making available the source code, and publishing the protocol, so that this capability can be added to any web service or application.
Thus comparisons to JotLive and Writely are somewhat orthagonal. They are web services. If they wanted to, there is no reason why they couldn't implement our source code as part of their product.
The goal of SynchroEdit is to offer the capabilitity of synchronous editing as broadly as possible.
