As Alm suggested, a logic-level MOSFET transistor is a perfect candidate for your application.
However, all depends on the application you have in mind:
a) If you already have a perfectly good DC Power Source (constant voltage), a MOSFET is all you need
b) If you don't have a DC power, have a look into the pre-made Point Of Load (POL) modules. Many of them support an ON/OFF control and with some luck, maybe some of them are able to be driven with a PWM signal. Then you can directly control the POL module
c) If you just want to drive some LEDs or such, most common LED drivers directly accept a PWM control input.
Cheers,
Dan