Here's the official Arduino Ethernet Shield With PoE Module
Using the Wiznet W5100, the Arduino Ethernet Shield allows an Arduino board to connect to the internet. This shield supports four simultaneous socket connections with TCP and UDP.
The latest revision of the shield adds a micro-SD card slot, which can be used to store files for serving over the network. It is compatible with the Arduino UNO, Arduino Duemilanove and Mega (using the Ethernet library).
A separate 3rd-party power-over-Ethernet (PoE) module is soldered to the board to provide power from a conventional twisted pair Category 5 Ethernet cable. It is IEEE802.3af compliant. Plug this bad-boy into a network with appropriate hardware, and it'll be totally powered by the network cable!