Developer website: Audacious Software
Cidney is an application for the Mac designed to be a smarter and more flexible replacement for your phone's ringer and Caller ID system. Using modern technologies such as Growl notifications and AppleScript, Cidney not only tells you who's calling, but can take action based on that information as well. For example, if you'd like to receive an e-mail whenever you miss a call at home, Cidney can send that for you.
Cidney also uses local peer-to-peer networking technology to let all of your computers know who is calling. Simply connect one computer to the your landline and that computer will tell the others on the network when you receive a call. Using this feature, you can receive Caller ID notifications on a any computer - just as if it were connected to the phone line itself.