Note for Drupal 7 users: this module is Drupal 6 only, you can try out the Taxonomy access fix module instead.
Taxonomy term lock adds a permission for each vocabulary, this permission allow users which does not have the "administer taxonomy" permission to add and reorder terms for this vocabulary.
Users with "administer taxonomy" permission will be able to lock some terms, prevent editing and prevent node to be tagged with those terms.
Locking is an optional feature, you can just use the module to assign individual administer terms for vocabulary N permissions (N is the id of that vocabulary, permissions strings uses the vid in order to ensure consistency if vocabulary name changes).
So this means that you can give certain users the ability to edit only certain vocabularies, but their edit powers are full add/edit/reorder/delete, unless you start locking things.
Note that menu items still need to be created with paths like "admin/content/taxonomy/N" (N is a taxonomy vid), permission are set and used with menu based access callback which ensure the full Drupal security checks at the lowest level possible.
This module is also able to hide vocabularies that users can't edit from the normal vocabulary list administration page, in order to ensure better security a nicer UI.