How can I send a mail message later? On Mountain Lion I want to compose a message, but rather than clicking "Send", I want to schedule a future delivery date and have the mail sent then without my taking action as long as the computer is still running. 

I'm currently using Apple mail with Google apps configured using IMAP syncing.

The answers here using Mail Scripts by Andreas Amann no longer works on Lion or Mountain Lion. Can something be done on 10.8 or later to automate sending Apple mail?
If you're using Gmail and are willing to use the (I think very awesome) web interface, check out Boomerang for Gmail. It does everything you're asking for.

EDIT: 

You can also use Automator to schedule (by delaying) when your message is sent. 

The Pause function is very helpful, and Apple has a whole section for Mail in the library. I would save it as an application and then run it with Spotlight or Alfred or Launchbar whenever you need it.