Can you turn on / off LEDs from a webpage (javascript) via a microcontroller (arduino)?
The software could combine the arduino examples blink with webserver. 

The hardware to run the web server on the arduino would be either the Ethernet or WiFi shield. Alternatively, you may want to run the server on another machine and use the USB serial link from that machine to the arduino to control whether the led is on.

The web server could set a variable when a url is posted to, and that variable could determine whether or not the blinking occurs. Ask at stack exchange or google for the javascript side to use  ajax to post a value to a url when the user does something, or use a simple form with checkboxes to start with.