Radio Shack #63-1110 "Mail Guard" page 318 in 2002 catalog on sale till end of april.
I know, you're thinking "so what, it beeps when the mail man opens the mail box, big deal". It can do more than that, read on!
For $10 you get a 320Mhz coded TX-RX, the TX is weatherproof! It detects light change and sends a coded signal to the RX, the RX beeps and sets a light on till you press reset. Up to 150 ft they say.
Here is what a true hacker sees:
Replace the photo resistor with a switch, or bypass that circuit (looks like a op-amp), use it like a alarm.
Replace the beeper or LED on the receiver with a relay or opto, the LED "latches" on till reset is pressed.
Connect the output on the receiver to my I-Opener parallel port, connect several RXs (what a deal $10!!!) and monitor the entire block's mail boxes and log the progress of the mail man as he drives down the street! (only kidding)
The output for the LED is probably 5V compatible, so connecting it to the parallel port should be easy, the beeper is a pezo so it probably is a 5V square wave and is easily picked up by software.
The chip inside has most of the "code" pins shorted to ground so you have more codes than the three if you don't mind cutting a few traces.
The switch only selects three with 2 chip pins, leaving one more code if you simply short two pins.
The receiver board is separate from the control board in the RX! It connects with three pins, power ground and signal, so you could have a PIC or Atmel decode any signal.
I think it's 320Mhz, I could be off by a few Mhz.
It's worth $10 just in parts!
12v wall wart transformer for RX and TX is sealed with a o ring, uses two 3V lithium CR2032 button cells, like motherboard type, comes with them.
I measured the distance and 150 ft is in open air, through walls it's about 100 ft or less.
Did I say it's only $10 on sale?
All Electronics in California used to sell a sensor with a coil, spring and magnet inside that would detect ground movement, you could connect it to the input for the photoresistor and bury it. When something walks over or a car drives by it would go off! The sensor was for oil companies doing ground testing for oil. I don't know if they still sell it.
Have your I-Opener click a web cam pic whenever it goes off!