Your eyes have cells which contain pigments which react upon exposure to single photons. Single photon signal is really, really noisy though and so your eye requires thousands of such signals to combine in spatially and temporally relevant patterns before it sends a signal to your brain.
Hah, cool finding. I was considering a situation more similar to "detecting one further photon of light from an already illuminated scene": just-noticeable difference instead of absolute threshold.
Back in high school we calculated the energy in a single photon by turning down the power on an LED until it almost went out. We recorded the amount of power, and figured out the energy in a photon within 15% accuracy.