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