Manage Taxonomy Trees
When working with taxonomy, especialy large vocabularies with deep hierarchies the standard taxonomy module isnt enough to manage your vocabularies. To maintain the vocabulary in drupals backend the most obvious choice is http://drupal.org/project/taxonomy_manager [1]
Some features:
Assigning taxonomy to nodes
The core taxonomy module only gives you the choice between a select field and an autocomplete field. The choice to extend this here is the http://drupal.org/project/hierarchical_select [2] , it allows you to select terms on nodes using a widget that selects the term by hierarchy level. So by selecting the a parent the next select field with only contain the children of the previously selected parent term. It also integrates with a large number of other modules.
Taxonomy permissions
http://drupal.org/project/taxonomy_delegate [3] lets you delegate other roles to administer certain vocabularies.
More than once the same vocabulary on your node
Ever wanted to add a taxonomy vocabulary more than once to a node under a different name. For eaxample you have a node you want to tag with a location vocabulary for places where the product is sold and use the same voc to indicate where the goods are produced. http://drupal.org/project/content_taxonomy [4] solves this problem by using the taxonomy in a cck field, you can add multiple cck fields of the same vocabulary on your node.
Import/Export taxonomy terms
The most commonly used module to import export taxonomy on drupal sites is http://drupal.org/project/taxonomy_csv [5] . Like the name says it creates a csv of your taxonomy which you can import on an other site.
Taxonomy and Access
Taxonomy can help you manage access too. http://drupal.org/project/taxonomy_access [6] makes nodes accessible using their taxonomy.
Other
Taxonomy could be used to build a menu(http://drupal.org/project/taxonomy_menu [7]), breadcrumbs(http://drupal.org/project/taxonomy_breadcrumb [8]), trails, ...
Links
[1] http://drupal.org/project/taxonomy_manager
[2] http://drupal.org/project/hierarchical_select
[3] http://drupal.org/project/taxonomy_delegate
[4] http://drupal.org/project/content_taxonomy
[5] http://drupal.org/project/taxonomy_csv
[6] http://drupal.org/project/taxonomy_access
[7] http://drupal.org/project/taxonomy_menu
[8] http://drupal.org/project/taxonomy_breadcrumb