For quantum-sized nanocrystalline silicon (nc-Si), various optical and electronic effects have been clarified in addition to a significant band gap widening. As typical examples of these induced effects, some emission properties of nanocrystalline porous silicon (PS) are described in this paper including the present status of application studies. The first one is electroluminescence (EL) of PS diodes. It is shown that following a drastic improvement in the external quantum and power efficiencies, stability has been significantly enhanced by the formation of covalent termination nc-Si surfaces. Next topic is the cold electron emission from PS diodes. When the nanostructure of the PS drift layer is appropriately controlled, injected electrons are accelerated ballistically toward the outer surface and emitted via tunneling through a thin-film top electrode perpendicular to the device surface as energetic electrons. As an efficient surface-emitting electron source, there are many advantages in this emitter over the conventional cold cathodes. The applicability of this emitter to either vacuum-type or solid-state flat-panel display is demonstrated. Finally, the usefulness of a PS device as a thermally induced ultrasonic emitter is presented on a basis of its fundamental characterizations. Technological potential of this emitter for functional acoustic devices is also discussed.