If you have your PC somewhere deep in your stuff or you simply want to impress someone, this project might interest you. Simply speaking this is a PC on trigger circuit activated with a knock. Knock detector is based on piezoelectric sensor which is nothing more than piezo speaker from some gadget. Attiny45 reads piezo signal using ADC and program detects if there is a required pattern (three knocks).
If MCU detects three knocks it then generates 0x29 keyboard code and sends using PS/2 interface to computer. In other end. PC has to be configured to boot on PS/2 keyboard input. The only thing is left – hide sensor under table and start doing magic.