This is a very simple module that adds a 'weight' field to the tables already used by the excellent Flag module.
This weight can then be used to provide ordering of flagged items. This module also provides a Views field which can be used to sort flagged items. One interesting use of this ability is to integrate with Draggable Views to create a user-sortable list of flagged items. See description of how to configure Flag Weights + Draggable Views (Drupal 6).