I've been trying to implement David Allen's Getting Things Done productivity system, in particular the to-do lists, for a little while. This hint isn't exclusive to that system -- it would work for any to-do list, or any document that a single user wants to edit from multiple locations, for that matter.
My main criteria for a useful to-do list were that the lists be available from any of my three computers at any time, and that they be as easy as possible to use (including drag-and-drop list ordering -- I can't stand having to set priority numbers!). For a while, I optimized on my first criteria by using an online wiki, but the editing wasn't nearly as drop-dead easy as a native Cocoa app. I considered using OmniOutliner, which has a much better interface, but keeping track of different versions of the to-do list would be more of a hassle than I wanted.
Finally I found a happy medium: an Applescript wrapper for OmniOutliner that will automatically sync to my main machine before and after I make changes.
To do this you will need:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20051117094952410