Tag Contact Via Link in Email
I would do this with a Drupal webform as follows:


Contact 1 - current user (if not logged in you'd need a checksum)
Contact 2 - the person to be tagged - set this as "existing contact" prepopulated from the url.
The tag will have to be preselected in the webform, so you'd have do clone this webform for each tag you want.
Add a tiny bit of javascript to the form to make it auto-submit.
On submit, redirect or display a success message.