Checking if the mail has been delivered can be a pain. To solve this problem, DIY blog LowPowerLab put together a simple notification system that texts you when new mail arrives.
To build this, he's using an Arduino clone of his own, a sensor, RF signal and a battery. With that, the system can send him a text message every time the mailbox is opened. It won't work for every type of mailbox, but the project could be extended for other applications. Head over to LowPowerLab for a full guide to make one for yourself.
MailboxNotifier — Moteino based mail event notifier [LowPowerLab via Hack a Day]