I’ve adapted the admin UI for tagging posts from the cat2tag plugin Dan mentioned earlier, as well as adding my own features, including support for adding tags on the fly that contain spaces, as well as auto-tagging. By this I mean that as I am typing (or if I open an old post for editing), the plugin will continually scan the text and automatically add any tags that are found in the text or title of the post.
This will make it very easy to go back and update older posts now, since I just have to open then save, since the post will automatically be scanned for tag matches, and I would just need to click to add one that isn’t specifically spelled out in the post, or type a new one quickly.