i built one a bit ago using an event (onselect or onchange?) handler on a regular dropdown that, when you click on an item, the handler removes that element from the dropdown options, and adds it to another ul/li list under the dropdown, with a little X to reverse the process

select2 does something similar but yea it's weird this isn't a built-in

maybe a dropdown with checklists?

And one with drag and drop reordering. or at least some form element that allows drag and drop reordering (ideally of arbitrary markup blocks)