Fargo

Not the movie, not the TV show. Elmer Masters of CALI fame has written a blogpost on Slaw about Fargo the Outliner. Elmer's recommendations are always useful, and Fargo looks like super useful web-based software that helps you easily create outlines on the fly. Not only that but it has some excellent additional features that let you, for example, easily turn your outline into a presentation, or set up a note/link blog to track interesting things on the internet.  Thanks Elmer!
p.s. You need to have a Dropbox account (they're free) to use Fargo.