Hey, <a href="https://mixmax.com/" rel="nofollow">https://mixmax.com/</a> has scheduling.<p><a href="http://www.boomeranggmail.com/" rel="nofollow">http://www.boomeranggmail.com/</a> has it as a main feature.<p>Also <a href="http://www.getsidekick.com/" rel="nofollow">http://www.getsidekick.com/</a> will add it soon.<p>If in doubt, check: <a href="https://zen.market/?q=schedule%20emails" rel="nofollow">https://zen.market/?q=schedule%20emails</a>
I highly recommend Mixmax (admittedly biased; co-founder here :-P). It offers a ton of free features beyond just scheduling emails. You also get unlimited email tracking for free and can put message enhancements in your email (things like link previews, polls, surveys, meeting invites).
I use my own simple google apps script. Has a benefit of working with any mail program: <a href="https://github.com/vitaly/gmail-later" rel="nofollow">https://github.com/vitaly/gmail-later</a><p>With gmail keyboard shortcuts work like a charm.
<a href="http://www.Yesware.com" rel="nofollow">http://www.Yesware.com</a>. I believe the free version only gives you so many though.