Thunderbird2 has tag functionality. However, the UI is not good enough. Methods to add tags are annoying if you have many tags. Attached tags are difficult to recognize. Tag Toolbar facilitates the tag functionality.
This module provides "suggested terms" for free-tagging Taxonomy fields based on terms already submitted. It replaces the description field on free-tagging fields with a clickable list of previously entered terms.
TagsFS is a file system for music files. It presents your mp3 and ogg files in a virtual tree based on the tags of the files.