If you’re looking for a customizable way to monitor your mailbox from afar, this DIY solution is ripe for additional electronics hacking.
At Make magazine they share a guide to creating your own remote mailbox notifier. The basis of the project is Arduino chips, RF modules, and a simple light sensor.
The basic project takes you from the chips to a functioning relay; from there you can customize with LEDs, audible alerts, or–for the truly ambitious–push notifications for your smart phone. For another look at snail mail notificaiton, check out this previous article.
Arduino Mail Notifier [Make]