I'm looking for a free (GPL or BSD-type license) outliner library that works in a browser.
It doesn't have to be too complicated, just allow keyboard control of collapsing and expanding items, and changing their order.
Anyone know a good library for this? Do any of the big UI component libraries have an outliner? If not, how would I go about writing it from scratch? (Eg. would it be better to start with jQuery? Or something else?)
The most difficult part will be enabling the keyboard to control the collapsing and expanding. The same functionality using only mouse events is pretty easy and you should find examples googling
The closest thing that comes to mind which is offered by most of the existing javascript libraries is the accordian. You would most likely have to some custom code to use keystrokes and appearance. Here's 10 Javascript Accordian scripts
Along with Serhii's suggestion you want to use the jquery-ui sortables feature. Check their demo code for examples. http://docs.jquery.com/UI/Sortable