This simple touch sensor project can be used to add a touch switch to your computer. It can detect when you touch a sensor connected to one of the sensor inputs. The project is built based on AVR AT90S2313 (you can replace it with ATtiny2313).
The basic idea of circuit is really simple. Make one pin output and another input. Connect a resistor between these pins. The resistor together with the human capacitance (about 100 pF) forms an RC network. The AVR set the output to low and then make a transition from 0V to +5V. 5 µs after this switch, the logic level at the input pin is sampled. If someone is touching the probe connected to the input pin, the capacitor (=human) will not be fully charged, and the input will be a digital 0 and vice versa. Download source code and schematic (Eagle format)