An outdoor motion sensor can be useful, either for hooking up to indoor/outdoor lights (that will turn on automatically when someone walks onto the property or drives up the driveway at night) or transmitting to a driveway alarm system (that rings a bell or chime when someone comes up the driveway).
You can get a simple outdoor motion sensor for about $20, but you may find it less troublesome to install a wireless sensor, such as this one.
The Optex Wireless Motion Detector costs $90, but you don't have to run any wires to install it, and it is supposed to be a little more reliable (larger sensitivity zone + it stands up well to the elements) than cheaper models I looked at.
Features include an adjustable field of view you can tailor for narrow areas or wide spaces, and it runs on a 9V battery, so it doesn't need to be placed anywhere near an electrical outlet.
You can get the outdoor motion sensor and a driveway alarm or lights at SmartHome:
