The Arduino Ethernet is a microcontroller board based on the Arduino Uno, and incorporating a WizNet W5100 TCP/IP Embedded Ethernet Controller.
It looks strange by having an ethernet jack where the USB port usually is, but the Arduino Ethernet (with PoE) still offers standard functionality from the same people who brought you the original Arduino! The PoE function lets you run power to the Arduino just using the ethernet cable (and compatible networking equipment).
It still has the 14 digital input/output pins, 6 analog inputs, a 16 MHz crystal oscillator, a power jack, an ICSP header, and a reset button.
Note: Pins 10, 11, 12 and 13 are reserved for interfacing with the Ethernet module and should not be used otherwise. This reduces the number of available pins to 9, with 4 available as PWM outputs.
An onboard microSD card reader, which can be used to store files for serving over the network, is accessible through the SD Library. Pin 10 is reserved for the Wiznet interface, SS for the SD card is on Pin 4.
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, and works with all compliant PoE injectors currently available.
Important: Programming-wise, you do lose the USB jack, but there still is the 6-pin serial programming header that is compatible with most standard USB to TTL converters, like our TTLyFTDI and SparkFun Basic Breakout. Or, use the 6-pin ICSP programming header (which, ummm, over-writes the bootloader). You cannot program it live over ethernet - you must use an comm-port adapter, or ICSP AVR programmer!
Official board release information is here.