Arduino Ethernet Shield W5100 is one of the most interesting expansion boards allows to control the Arduino board via LAN or Internet. This can be useful, if you need to get some information from a network, place your own web server. Or use Arduino for control a network.
On this Ethernet Shield
Price : $4.99
original site : http://robotdyn.com/catalog/shields/ethernet_shield_w5100_for_arduino_uno_mega/