If I wanted to buy a receiver that measures the energy of incoming photons, what is, not theoretically, nowadays the lowest energy that can be measured (in joule)?
It depends on what you mean by "a receiver that measures the energy of incoming photons"
In the optical world, the usual way to do this is (more-or-less) build an external tunable filter, in front of a detector that can receive a wide range of photon energies. Or make a device that delivers photons of different energy to different elements in an array of detectors (which aren't themselves energy-selective). Depending on the details of how its constructed, it could be called a spectometer, a spectrophotometer or a optical spectrum analyzer.
But again, the actual detector isn't highly selective of the photon energies it detects, nor is it able to measure the energy of the photons it receives. Typically, photonic detectors (such as photodiodes in their various forms, or phototubes) have a minimum photon energy they can detect (due to the photoelectric effect) and very roughly, produce an output signal proportional to the number of photons received for any photon energy above that threshold. The upper limit on the receivable photon energy will typically depend on engineering details, the optics collecting light into the detector, etc.
AFAIK, the photonic detectors able to detect the lowest photon energies are quantum cascade detectors, aka inter-subband detectors. These use a stack of quantum wells in a semiconductor heterostructure to allow detecting fairly low-energy IR photons. According to this abstract, as of 2009, inter-subband detectors were able to detect wavelengths as long as $87\ \rm\mu m$, corresponding to a photon energy of about 0.015 eV.
More esoteric devices involving superconductors and quantum capacitance can, as of 2017, detect frequencies as low as 1 THz, corresponding to ~0.0006 eV (hat tip to S. McGrew for finding this reference).
But, of course, radio antennas also in some sense detect radio photons, which have much lower energies than the long IR detectable by inter-subband detectors. However they'll only usefully detect radio waves containing vast numbers of photons. I've assumed you're looking for a device that can detect reasonably countable numbers of photons (10's or 1000's per measurement) rather than the trillions or more that might be involved in detecting a radio wave, so I won't work out what is the longest radio wavelength detectable with current technology.