A stable variable power supply with an adjustable output voltage from 5 volts to 30 volts can be easily constructed with the regulator ICs UA78G or UA79G. These ICs differ from the common three-terminal regulator since their output voltages are adjustable by a voltage level at their control inputs. The maximum current delivered by these [...]

This simple circuit, consisting of two LEDs and a photo diode, generates a negative voltage with a current level of a couple milliamps. It is ideal for supplying a negative rail to low power “rail to rail” op amp circuits [...]
Source: http://www.discovercircuits.com/DJ-Circuits/negsupply2.htm

Sometimes you need a simple negative power supply. The best example is the contrast PSU for common small LCD device. Building -5V from a battery or a wallmart supply isn’t really easy.Here is a simple circuit that is able to provide -5V, -12V, -15V. The circuit used the MAX 764/765/766 series. You can simply change [...]