You can control almost anything with an Arduino, and via an Ethernet connection do so from afar. One simple and interesting example of this has been created by Dan Nixon who found some inexpensive RGB LED strips, and made a system that allows their control from over the Internet by an Android app on his smartphone.
Apart from an Ethernet-enabled Arduino board the project requires a power supply for the LED strips and some NPN switching transistors to allow control of a higher current from the Arduino's digital output pins. You could build such a system in a couple of hours, and then have some futuristic light controls as demonstrated in the following video:
If you're looking to make more creative designs with RGB LEDs, consider the Freetronics RGBLED: full colour module. It includes a bright RGB LED on the top of the board and a WS2801 constant-current, addressable, multi-channel LED driver on the back. This smart module can be daisy-chained, so you can connect a number of these together in a string and drive each of the module colours individually from your microcontroller. For more information and to order, visit the product page