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.