Firefox/Chrome: Right Inbox is an extension for Chrome and Firefox that works with your Gmail account to schedule emails for sending later, much like Boomerang, but free and simpler.
Scheduling emails could be helpful for quite a few reasons. One example is that you might know the recipient gets to their desk at around 10am, so you could schedule your email to be delivered at some point around that time. Otherwise, your important message could become buried by all of the other emails that the person has gotten since you wrote it.
The add-on places a "Send Later" button in Gmail with options to schedule the email in 1, 2, or 4 hours or select a specific date and time and, optionally, time zone. It's very straightforward and intuitive—and useful. Use it to send yourself reminders or send emails to others at just the right time.
Overall the app is lightweight and definitely worth installing and you might start seeing faster responses from the people you’re emailing if you start thinking about the best time to fire that important message off. If you want this feature and a whole host of others, definitely check out Boomerang, which is a paid alternative.