I've attached two LEDs, a dial, a light sensor, and a passive infrared motion detector to the Arduino Ethernet. I've got it running a telnet server and a web server. I can telnet to the little guy and instruct it to turn the LEDs on or off. Or point a web browser at it, to see the current status of lights and sensors. It tries to return JSON, but my structure might not be entirely clean. Also, it's behind a few layers of NAT, so not yet publicly available. (Soon!)
It's all stand-alone, which is pretty kewl. Give it a connection and a few volts -- it'll happily start up and begin sensing things and controlling its lights. You can imagine keeping these in a data center, with DHCP and a dynamic DNS client. Plug it in, it reports its IP to you and awaits queries about the environment, or commands for the LEDs. (Or reads sensors and sets LEDs by its own logic, while it waits for a human.)
The whole thing -- board, power adapter, cable, sensors, lights -- is well under a hundred bucks!