Although I’ve long thought collaborative realtime editing a useful concept, I’d never tried it hands on until a month ago when Christopher Allen got me to use MoonEdit. Very useful, but not free software and as far as I know no OS X support.
At the iCommons summit someone wanted participants to collaboratively take notes on a wiki. Wikis so far are great for asynchronous distributed collaboration, unworkable for synchronous distributed collaboration. Luis Villa pointed out ☠, an experimental collaborative realtime editor hosted in MediaWiki pages (but not very tightly integrated with MediaWiki–only new chunks of a page may be collaboratively edited in realtime, and the server side of this AJAX application is a separate server written in Java). Great idea, and the whiteboarding sub-application is neat, but I’d really prefer to be able to collaboratively edit an entire page.
Seems to me that almost any content creation application could benefit from optional realtime collaboration, legacy desktop applications included. Word processors could get their first useful new feature in many years.
Update: Also see ting-wiki for a hybrid web-local editor approach.