Is your mailbox standing away from your house? Maybe you would like to know exactly when mail arrives? No problem at all. The guy called newbie123 in AVRfreaks have build a nice system which rings a buzzer when mailbox is opened.

System consist of two parts. Transmitter and receiver. Both parts are controlled by AT90S2313 AVR microcontroller, that can be easily replaced by Attiny2313. Messenger uses the TLP434A transmitter and the RLP434A receiver from SparkFun. The transmitter is mounted inside a small plastic radio shack box. Transmitter is placed at the back of the mailbox so it does not interfere with anything. The antenna is routed down through the bottom and just sticks down under the mailbox. When the mail man opens the door to the mailbox, the ground wire is connected to the circuit and the transmitter powers up, transmitting the 4 byte packet to the receiver mounted inside the house. Cool Toy.