Android: Boomerang, the Gmail app and extension that allows you to schedule emails for a later date and time, has come out with its own dedicated app for Android.
The app offers a few options above and beyond what Google's offering provides. In addition to being able to schedule emails, you can also snooze received messages and track responses to emails you've sent. Boomerang works with Gmail and Google Apps accounts, but unfortunately it doesn't support any other email clients just yet.
Boomerang (free) [Google Play]