Privicons for Gmail is one secretive add-on. For Gmail users who prize privacy, that's a good thing. Once you've installed the free Chrome add-on, you can click the Add Privicon button in your Gmail taskbar and add color-coded privacy indicators to emails, alerting recipients to their sensitivity.
Self-explanatory options like "Keep private" and "Don't print" provide clear indicators that the email is best kept quiet. "Delete after reading/X days" lets you specify how long a reader should keep the communication. For those worried about going on the record, "Don't attribute" tells a recipient to keep your words anonymous. If you and a recipient have a shared circle of work or personal associates, the "Keep internal" tag encourages them to keep your communication in the family.