taxonomy_similar will display a "similar tags" screen anytime content has been created or updated with tags from a "Free tagging" vocabulary. This module requires no configuration. Install it, enable it, and create or edit content that has a "Free tagging" vocabulary assigned to it. The "similar tags" screen is entirely optional to the content creator - by the time it appears, the new or edited content has already been saved to the database. The content creator can edit his tags, or simply move on.