This generation is spoiled with its methods for instant communication. Emails, texts and IMs arrive mere seconds after they were sent, and even come with a wonderful notification sound. However, if you still use snail mail for anything important, it can take days to show up in your mailbox, and there is no notification whatsoever. Well, not unless you decide to make one yourself, that is.
There is a great guide over at MAKE that can tell you how to turn two Arduinos, a light sensor and a couple of RF modules into a snail male notifier. In roughly an hour, you should be able to craft a sensor that lets you know every time the lid to your mailbox has been opened. You just have to hope that your neighborhood mailman doesn’t see it and freak out too much.