The invention of a practical, long-lasting lightbulb by Edison and Swan in 1879 was a turning point in the history of human civilization. Instead of expensive candles giving off weak light, you could have cheap lightbulbs that brighten up a room like daylight.
Of course, the lightbulb has undergone some changes recently. Power-hungry incandescent bulbs have been replaced by more efficient and longer-lasting CFLs and LEDs. However, incandescent bulbs might be on the way back, just not how you'd expect.
If you remember, incandescent bulbs work by sending an electrical current along a tungsten filament. The filament heats up to the point that it produces visible light. Unfortunately, it wastes a lot of electricity as heat.
You also can't scale it down very far because the filaments burn out faster. Two words: Christmas lights. If I never have to hunt down burned out bulbs again, I'll be happy.