Android: We've seen SMS schedulers before, but Schemes takes things three steps further by allowing you to send scheduled tweets, Facebook posts and emails (via Gmail) as well.
The app can send your message to any combination of the four services at once. Just pick the date, time and fill in the requisite information. As long as your device is connected to the internet when your message is scheduled to go up, everything should be fine. You can also elect to receive a confirmation notification if you'd like to be extra sure that your correspondece gets through.