How is the photoelectric effect used today?
One use is LDRs (Light Dependent Resistors)
As you know, the photoelectric effect relates to the emission of electrons from a metal when light of a minimum frequency hits the metal. An LDR contains a semiconductor, which is mainly made up of some sort of metal, the metal can determine the minimum frequency of light needed, due to the work function.
By having light incident on the surface, more fre flowing electrons can be released which increases the current, and reduces the resistance, less stationary electrons in the way. The greater the light intensity, the more incident photons, the more electrons emitted, the less the resistance.
This process also works in photocells to make circuits only work when it is bright, or in the day time.