Inline Tag Thing allows you to add and remove tags on posts, inline, using magical AJAX powers.
http://downloads.wordpress.org/plugin/inline-tag-thing.zip has the latest version.
Drop the plugin file into your wordpress plugins folder. The plugin expects to be installed in /wp-content/plugins/InlineTagThing/
Enable the plugin.
When you view a post that you are able to edit, a tag edit box will display. Entering a comma separated list of tags in the text box, then clicking the “+” button will add those tags to the post. Clicking the “-” link beside a tag will remove it. The list of Currently Assigned tags list reflects the tags which are assigned to the current post. When tags are added or deleted, this list will update to reflect that change. Tags which are displayed elsewhere for the post will not reflect the changes until you reload the page.