Boomerang for Gmail is a Firefox/Chrome extension that allows you to schedule send an email for the perfect time.
One of my faves! Try Boomerang for Gmail. You can schedule emails to send at any time and track responses to follow up.
With Boomerang, you can write an email when it’s convenient, then schedule it to send automatically at the perfect time. Just write the message as you normally would, then click the Send Later button.
From the drop down menu, you can schedule sending time with either the handy calendar picker or text box; our smart menus display your most frequently used scheduling times for easy access. Boomerang will then automatically send your message at the chosen time, without you having to worry about it.