If you want a wireless one, I currently have the Dakota Alert WPA-3000 installed.
It has the usual driveway probe that you bury and has 50' of cable that runs to the transmitter. The weatherproof transmitter box is a little big but the 50' cable allows you to run it off to the side to hide in the trees or bushes. They also make a wooden cover now that goes over the transmitter that makes it look like a birdhouse!
The receiver that comes with the package has 4 channels so you can add 3 other wireless devices (driveway probes, PIR, hose sensor, dry contact switch transmitter). Each channel has an associated relay (mine is connected to an Elk M1). The receiver also has an audible alert if you want that.
The claimed range is 3000'. I never tested it but currently my driveway transmitter is over 200' away from the receiver inside my equip. room with the house in between.
It took a little bit of playing around to get the sensitivity set just right to avoid false alerts but still sense any vehicle coming down the drive.
I'm getting ready to hook up a mag contact to their dry contact transmitter and use another channel on the receiver to sense when my driveway gate is opened.
Also the new SimpleHomeNet EZSnsRF wireless receiver/Insteon controller works with any of these Dakota Alert series 3000 wireless sensors if you don't have an automation controller.