TestSubject - Test how your email subject line displays on mobile devices.
Mailcastr - Create email signature that tells when your email is read.✉️
analyze.email - Test the spam-y-ness of your emails
Ugly Email for FireFox - Get back your email privacy, block email tracking
Rename Email - Easily change the subject of any email, by cloudHQ
PixelBlock - Block email open tracking inside of Gmail