Posted by juan_g on August 12, 2009 at 11:55pm
For a drupal.org's list ordered by popularity, that is by usage statistics in descending order, see: Taxonomy modules. Currently, there are 218 contributed modules related to the core Taxonomy module, 134 of them with a Drupal 6.x version available.
They include, as the most used taxonomy modules: Tagadelic, Taxonomy Menu, Content Taxonomy, Taxonomy Manager, Feed Element Mapper, Taxonomy Breadcrumb, Menu Trails, Taxonomy Image, Taxonomy Access Control, etc.
(There is also a wiki page in this group on key taxonomy modules).
Note: The old link to taxonomy contributed modules (in the mission statement of this group) is not working correctly at this moment: it shows all modules, not just the taxonomy ones. I think it should be updated.