Carnivore is a surveillance tool for data networks. At the heart of the project is CarnivorePE, a software application that listens to all Internet traffic (email, web surfing, etc.) on a specific local network. Next, CarnivorePE serves this data stream to interfaces called "clients." These clients are designed to animate, diagnose, or interpret the network traffic in various ways. Use CarnivorePE to run Carnivore clients from your own desktop, or use it to make your own clients.
http://r-s-g.org/carnivore/