Chrome: We've discussed plenty of great tools to clean up your Gmail. If you're looking for an all-in-one tool with a lot of useful features, mxHero Toolbox delivers. In one package, mxHero offers send later, self-destructing email, attachment tracking, notifications when your message is opened, and much more.
The video above does a pretty good job of explaining how the tool works. Once installed, compose your Gmail messages normally, then click the mxHero button to open up the toolbox. From there, you can choose the mxHero "apps" you want enabled for that message. You can schedule the message to send at a specific time, use attachment tracking to get a notification when the recipient opens and reads whatever you've included with your message, set your email to "self destruct" five minutes after it's been read, or use "private delivery" to compose your message to a group but only show the recipient's name in the To field (which kills the urge to hit reply-all,).
You can read more about the individual toolboxes in the mxHero toolbox guide. You can grab mxHero Toolbox from the Chrome Web Store at the link below, and it's free for individual users (and up to three users on your own domain).