Firefox/Chrome: Right Inbox is an extension for Chrome and Firefox that works with your Gmail account to schedule emails for sending later. It's like previously mentioned Boomerang, but free and simpler.
The add-on places a "Send Later" button in Gmail with options to schedule the email for sending in 1, 2 or 4 hours, or at a specific date and time. It's very straightforward and intuitive — and useful. Use it to send yourself reminders or send emails to others at just the right time.
Boomerang has more features like follow-up reminders if no one responds, but it is only free for the first few emails of the month. Right Inbox is free throughout the beta and says it will always have a free starter package, so if all you need is simple email scheduling, this is an easy-to-use Boomerang alternative.