How may an Apple's USB port be used to control simple electronic devices e.g. LED(s)? How may an Apple's USB port be used to control simple electronic devices e.g. LED(s) ?

I'm happy for any workable answer, but hopefully someone may know of a link to a supplier of a cheap USB interface that can control a LED (light emitting diode) when plugged into an Apple, and some simple script/application is run to change the LED on/off.
Arduino ftw

http://en.wikipedia.org/wiki/Arduino

You can get simpler than this, and just use the power from the usb port to power a breadboard LED device. You should really have a basic understanding of electronics, be able to read a schematic, etc., before you do this. Its not that you will electrocute yourself, but you may lose the use of the usb port.