Boomerang for Gmail from Baydin lets you schedule email to send later. It installs as an extension in Firefox or Chrome and adds a big "Send Later" button to the top of your compose window. Use it to schedule email to go out in an hour, or tomorrow, or next month - anytime in the future, even if your computer is offline or shut down completely.
Boomerang adds a 'send later' button that lets you write an email now and schedule it to be sent automatically later. You can use a Calendar, or simply type in date and time. It will also take messages out of your inbox until you actually need them, returnign the message to your inbox, marked unread, starred, or even at the top of your message list, at a time of your choosing. Other productivity options include reminders if an email hasn't been replied to within a certain length of time. There's also a mobile version of Boomerang for Gmail.
To use Boomerang, you need to give the service access to your Gmail account. The company says it only accesses headers, but technically could access entire messages. And in some circumstances even message headers can contain sensitive information. So you'll want to be careful using Boomerang with sensitive information or in regulated environments.