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This is a combination (replacement) of two modules - the first being cck_taxonomy, the second being taxonomy_super_select.
What this does is present a multiselect taxonomy as a hierarchy of checkboxes. Currently, the single select (radio button) portion is not working, so don't use it. This is very close to working, but I don't know why it isn't! This is driving me mad! See this post to pitch in and help: http://drupal.org/node/227306
It also presents a novel interface for free tagging which allows free tags to be stored hierarchically just like other taxonomies. New features: add multiple terms at once via comma-separated list; disallow creation of top level branches; disallow creation of any level branch; alphabetizing now works correctly
A use case for the first branch restriction feature is where you define States as top level branches and you want users to populate them with city terms. Naturally, you don't want users to add States!
A use case for the second branch restriction is where you pre-define a multi-depth vocabulary, but only want users to enter new leaf terms as free tags.
Can't decide which taxonomy module to use? Look at this comparison. Note that the description of this module in that post may not be up-to-date. Refer to this page and the README for clarification.
Contact me via my Drupal profile if you'd like to hire me to set this module up, modify it for your particular use, or other Drupal/PHP work.