Have emails return to your inbox at a specified day and time. Stop thinking about things until your past self decides you need to be thinking about them again. Keep a clean inbox. Want to reply to a letter but don't have time until after work? Boomerang it to tonight, and you'll get a reminder when it lands in your inbox and you have time to take care of it.
Free for the first 10/emails a month, $5/month for unlimited emails. I pay the $5 and it's very worth it.
I added Boomerang based on your recommendation. I already use Mailbox for iPhone to schedule emails to reappear at some given time. I've used Boomerang primarily to delay the sending of an e-mail ("Send Later") and to schedule emails to reappear in my inbox if I don't get a reply. I find "Send Later" particularly useful, since it removes the rationalization that "I'll compose this e-mail later, since I can't send it out until later".
I have often benefited from recommendations for Things I Didn't Know I Wanted.
Given that Less Wrong is a community of unusually intelligent, critical, and self-improvement-focused people, I suspect we can generate a pretty helpful thread of product recommendations — perhaps even a monthly thread of product recommendations.
Rules: