From the JSTOR Update Blog:
Students and researchers can use the tool to organize [and annotate] citations and draft paper and project outlines without leaving JSTOR.
This is the latest feature in a planned series of MyJSTOR account enhancements.
The outline builder includes a frequent auto-save feature. You can choose to stop and come back later – you won’t lose your work if you navigate away before downloading the outline.
Finally, JSTOR tells us that allowing Outline Builder users to share and collaborate items is currently under consideration.
UPDATE March 21