Firefox/Chrome: RightInbox is an awesome tool for boosting Gmail's capabilities. The extension when it first launched was designed to schedule email for sending later, but RightInbox since then has added even more useful features: email tracking, link click tracking and email reminders.
Once you connect the add-on to your Gmail account, you'll find new buttons in Gmail for scheduling an email to be sent later, tracking an email you compose, and adding reminders to an email.
With the tracking, you don't have to rely on read receipts. RightInbox sends you an email telling you when the recipient opened your message and if any links in your email were clicked. It'll even tell you where the person was when your email was opened.
The email reminder feature lets you add a handy reminder at a specific time or if you don't get a response to your email. You can also set the email to go to the top of your inbox.
With these new features and the send later feature, RightInbox really improves how you can work with Gmail.