It is of course the venerable Illuminated rocker
http://www.cnet.co.uk/i/c/blg/cat/blog/off_switches/3.jpg
All software designs should try to carry over as many as possible of its
excellent qualities:
- Two distinct states
- It is immediately clear if it is "on" or "off"
- Audible and tactile feedback
- Clear and understandable marking
Plus it adheres to standards.