The Adafruit 12V Bias Voltage Boost Converter uses our new favorite mini-booster, the TI TPS61040, to generate a 12V DC from as little as 3V input.
Adafruit use this chip a ton in their OLED boards, to generate the 12V bias voltage needed for the displays to function. But there's probably many other times and reasons you may want a high-ish voltage, such as driving an LED string backlight or re-programming FLASH or EEPROM memories.
Note that this board isn't good for high power driving, like solenoids or motors or long analog LED strips. You can get about 40mA out max. But definitely easier than getting a separate 12V supply when just a small amount of current is needed and you've already got 3.3 or 5V on the board.
You get one fully assembled breakout with a TPS61040 and required passive components, and a small bit of header. Give 2 to 6V DC on the IN and Ground pins. Then you'll have 12V on the 12V output pin. The Enable pin can be pulled low to disable the output.