Sick of walking over to the mailbox only to find it empty? Adafruit has a guide for building a system where you'll get notified when the mail arrives.
The system here uses a couple XBees pieces, the Flora and a bunch of other random parts. When all is said and done, a magnet triggers a switch that lights up, then sends a message to you telling you that you've got mail. It's a pretty complicated solution for something that isn't a huge problem, but it's fun nonetheless.
Mailbox Notification Service [Adafruit]