There are some cool and simple applications for doing collaborative editing on Mac OS X: the programmer-focused Hydra (seeking a new name), and the rich text (and seemingly science/math) document focused iStorm. What the world needs now, is for OmniOutliner to bring this same functionality to outlines!
OmniOutliner is my favorite outliner. It has a lot of features, but keeps its focus on being an outliner, instead of being an outliner, a diagram editor (OmniGraffle can open OmniOutliner outlines and make diagrams out of them), and a presentation tool (apparently, Keynote and OmniOutliner can work well together though). OmniOutliner, thus, remains simple, fast, elegant, and surprisingly flexible. Now all I need to do is be able to share outlines with a coworker - either in read only mode ("here's what I'm doing"), or in collaborative editing mode ("here's what we need to do to get this project done"). I'd be happy enough with the first, and overjoyed at the second.
Dave Winer added a feature to Radio Userland a couple of years ago called instant outlining. It sortof combined instant messaging with Radio's outliner. You could subscribe to other outlines and read them in place, and get notified whenever a subscription changed. Since outliners are a great way to both document what you need to do and what you have done, this was a great concept. I'm not sure whatever became of it. I ultimately abandoned Radio. And I really did not like the user interface of its outliner - it was slow and cumbersome and not terribly OS X savvy, compared to OmniOutliner which I was already familiar with. And I must say, I've never been able to get along with Frontier for any major stretch of time - it just didn't work with my brain, even though it's heavily outliner based and I do love outliners. But it was (and still is) a good idea. It satisfies the read-only shared outliner requirement.
I know people who use the outline modes available for Emacs to maintain a personal log that spans many years. I have a similar OmniOutliner outline, DOING!, that I just put back on my Mac OS X dock. Some of it is organized by date (but that ends mid-december 2002), some is organized by project. Either way it's grouped, it's still nice to go back and look at what's been done - and also to see the thought processes applied to certain problems.
Now if only I could share...easily.