Photon grenades were short-range, variable-yield explosive devices utilized by various cultures.
In 2367, Lieutenant Commander Geordi La Forge suggested transporting photon grenades down to the underground Turkana City in order to incapacitate members of the Alliance, who were holding two Starfleet officers hostage. He noted that, set to minimum yield, the grenades would effectively stun their targets. (TNG: "Legacy")
In 2372, after it was discovered that Changelings had infiltrated the Federation, Admiral Leyton began maintaining stockpiles of offensive and defensive technologies, including phaser rifles, personal force fields, and photon grenades, in preparation for his planned military coup of the Federation. (DS9: "Homefront")
In 2375, Tuvok, Tom Paris, The Doctor, and Noss came under attack from photon grenades while trapped in a subspace pocket. A group of marauders used the grenades in an attempt to disable the force field surrounding their ship, in an effort to steal their food and supplies. The group was rescued by the USS Voyager before the marauders could gain access. (VOY: "Gravity")
In the 31st century, Vaskan agitators attacked the Museum of Kyrian Heritage with photon grenades, after a back-up copy of The Doctor gave accounts that the Vaskans were not the instigators of the Great War, as the Kyrians had long maintained. (VOY: "Living Witness")