Arduino uno, VIN and 12 volts
Possibly. It depends exactly what it is you want to drive.

When you connect 12V to the barrel jack you get about 11.3V out of the VIN pin. This is because there is a diode in series with the barrel jack to protect against reverse polarity connections. This loses some voltage (could be as much as 0.7V).

Also that diode is rated at 1A maximum. So you can't drive anything that would need more than 1A (including the current needed by the Arduino and shield components).