How many subatomic particles can absorb/emit photons? Is the electron the only subatomic particle that can absorb and emit a photon?
 A: The photon couples to all particles with electric charge or magnetic moment. This includes all of the quarks, the charged leptons $e,\mu,\tau$, and their antiparticles. It also includes particles composed of quarks and charged leptons: the proton and neutron (though the neutron only magnetically), the charged mesons, etc. Many electrically neutral mesons, like the $\pi^0$, decay to states of only photons.
I'm not sure whether there is a good limit on the magnetic moment of the neutrino; I think it's connected to the question of whether the neutrino-antineutrino relationship is as described by Dirac, like the charged leptons and quarks, or as suggested by Majorana, with the neutrino and antineutrino the same particle.
A: If you are interested [let me know], I can explain to you exactly why this occurs (via quark annihilation) by using Feynman diagrams. Sub atomic particles that decay into photons include:
As you already know, the leptons (which includes the electron and various neutrinos) are mediated by photons.
Mesons: The neutral pion (which can decay into 2 photons), the neutral eta particle (can decay into 2 photons)
Baryons: The neutral sigma (which is like an excited state of the neutral lambda particle (decays into a a neutral lambda and a photon).
There are many more, but I cannot think of more off the top of my head (the truly intriguing aspect is which decays do NOT occur, even though they theoretically could occur). [I posted them at the bottom after an edit]
In the center of momentum of the 2 photon decays, each photon goes off in opposite directions with half of the total energy of the original particle. In the center of momentum of the 1 photon decays, the photon and the new particle go off in opposite directions, and the photon carries the difference of the rest masses between the two particles (note here that I say rest energy, because you do not have to add any energy to an accelerator for a decay to occur, nor can you ever add enough energy to make a decay occur.
My sincere apologies for misinterpreting your question in the comments. Hopefully this will give you what you need to research more photon particle decay modes (which are very intriguing).
Here are links to the wikipedia pages where you will have access to all of the particle decay modes:
Leptons : http://en.wikipedia.org/wiki/Lepton
Mesons: http://en.wikipedia.org/wiki/List_of_mesons
Baryons: http://en.wikipedia.org/wiki/List_of_baryons
