The following modules are grouped according to the most recent version of Drupal for which the module has a version (which may only be a dev version).
Drupal 7
http://drupal.org/project/lineage (adds hierarchical taxonomy support to views)
http://drupal.org/project/hierarchical_select
http://drupal.org/project/shs
http://drupal.org/project/hierarchical_term_formatter
http://drupal.org/project/taxonomy_menu
http://drupal.org/project/views_tree
http://drupal.org/project/term_reference_tree
Drupal 6
http://drupal.org/project/archive_by_terms
http://drupal.org/project/sitemenu
http://drupal.org/project/taxonomy_treemenu (unmaintained since August 4, 2010)
http://drupal.org/project/taxonomyblocks
http://drupal.org/project/tinytax
http://drupal.org/project/vocabindex