Apart from the photon, another known particle that is considered massless is the gluon. Gluons are elementary particles that mediate the strong force, which is responsible for binding quarks together within atomic nuclei. Gluons, like photons, are gauge bosons, which means they are carriers of a fundamental force. The strong force mediated by gluons is specifically associated with the interactions between quarks, and gluons themselves do not possess mass.
It's important to note that the concept of masslessness in the context of elementary particles can be complex due to the interaction with the Higgs field and the mechanism of particle mass generation. The Higgs mechanism is responsible for providing mass to certain particles, including the W and Z bosons, while other particles like photons and gluons are unaffected by this mechanism and remain massless.