Taxonomy Multi Editor

A mass category editor.
Allows any nodes shown on admin/content/node to have terms added or removed in bulk, using the normal 'node operations' selector.
Also provides a tab to view/edit all nodes terms individually.

With an image thumbnail, this can also be used for sorting uploaded images into galleries quickly.

Consider this module deprecated in favor of Views Bulk Operations - which offers a more cohesive interface to pretty much the same job
You may also experiment with editview or similar for mass updates.

Once installed, you will see new options 'assign to taxonomy term' and 'remove taxonomy term' in the content management screen, and a new tab alongside that page for a per-node view of similar operations.

For those that find the need, Views Bulk Operations is suggested as a more powerful utility to perform actions on many nodes at once, based on the select filters of 'Views'. Some assembly required.

See also taxonomy_node_operations (Jul 2007) which advertises similar functionality.