What is the smallest unit of energy?

We have Planck's constant h in joule-seconds, and for an oscillator operating at frequency f, the smallest unit of energy it can emit or absorb is hf. Okay, but in theory, by reducing the frequency f to ever smaller values, we produce ever smaller units of energy, eventually attaining infinitesimal values as f approaches zero. But isn't there a "smallest unit of energy," a value given in joules, not joule-seconds? What is the smallest "quantum of energy," assuming there is such a thing?