Programmer Joseph Thai has made a Super Mario Bros.-themed doorbell made mostly out of a PIC16F628A microcontroller connected to a dual 7-segment LED display, and a PIC16F687 microcontroller connected to a MCP4822 DAC and a 25LC1024 EEPROM. Thai has posted a technical breakdown of all this on his website.
Basically, every time a guest presses the coin button, the counter displayed will increment and the coin sound from Mario will be played. As a special bonus, every 10 coins will cause the life-up sound to be played, and every 100 coins will cause the mushroom upgrade sound to be played. Technically, the life-up sound should have been for every 100 coins, but the mushroom upgrade sound was so obnoxiously loud and long, that I switched those two clips.
image via Joseph Thai