Hello Jean Claude,
what about offering a small board for easy interfacing between e.g. 5V microcontrollers and peripherial things like e.g. sensors operating in 3V range only?
A similar board is available from Sparkfun, but at least according to my meaning, for universallity it would be better, having 4 bidirectional level converters ( according to: http://www.standardics.nxp.com/support/documents/i2c/pdf/an97055.pdf ) and also a small regulator for output voltages around 3V ( like e.g. LM317T ) on board.
Klaus