Chrome: Scheduling outgoing Gmail messages isn't exactly new, but there's always room for improvement. SndLater makes it easy to schedule an email for later, and it also allows you to remind yourself to check in on a conversation after a set amount of time.
The extension adds a "Send Later" button at the bottom of the email compose window. You can use natural language times to designate when you'd like to send a particular email (such as "tomorrow at noon"). You can also get a reminder for any email conversation. When viewing a message, click the clock icon at the top and choose when you want to receive a reminder. You can even choose to only get a reminder if no one replies. Very handy for that important question you need to make sure you get an answer to.