I'm a big fan of outliners-- a substantial portion of my life lives in shadowplan, the palm/windows/osx outliner--and I'm always looking at interesting new examples to play with. (As an aside, there is a fantastic series of articles running in About this Particular Macintosh called About this Particular Outliner that anyone even mildly interested in outliners should check out.) I've been tinkering with a couple of web-based tools lately: tadalist and sproutliner. They're both well designed with clean, easy-to-understand interfaces, but sproutliner wins on features - it's actually an outliner whereas tadalist is, as the name signifies, just a way to make lists. Anyway both are free and well worth checking out.