Dave Jones at the EEVBlog covered voltage doublers in a recent episode. His latest tutorial covers a variation of the technique, the voltage inverter. This is excellent technique to have at hand when you need a few milliamps of negative voltage for a circuit, perhaps for an op-amp circuit or to be able to adjust down to zero volts with an LM317 adjustable voltage regulator.
Dave clearly explains the details in this entertaining tutorial. It's recommended watching!