(covers information from several alternate timelines)
A stun grenade was a hand-thrown weapon utilized by both Starfleet and MACOs in the 22nd century. Designed with a time-delay setting, stun grenades were capable of incapacitating the enemy without causing permanent physical damage, in much the same way as a phase pistol set to stun.
In 2151, Subcommander T'Pol, first officer of the Enterprise NX-01, suggested using stun grenades to forcibly remove the descendants of Human colonists on planet Terra Nova, known as "Novans," from the tunnels which they had established beneath the surface. (ENT: "Terra Nova")
The next year, Captain Archer, Commander Tucker, and T'Pol, utilized stun grenades in a raid of a Suliban Stealth Cruiser, during their attempt to obtain evidence that the Suliban Cabal had framed them for the destruction of a colony at Paraagan II. (ENT: "Shockwave")
Shortly into their mission in the Delphic Expanse, Enterprise was raided by Osaarian pirates. During their attempts to subdue the Osaarians, Captain Archer ordered a MACO to utilize a stun grenade to neutralize one of them in the armory, as using their phase weapons in such close proximity to armed photonic torpedoes was too dangerous. The stun grenade proved ineffective, however, as the Osaarian was able to beam away before it went off. Amongst other items stolen by the pirates were two dozen stun grenades. (ENT: "Anomaly")
Later that year, Major Hayes and a team of MACOs used a stun grenade to incapacitate two Xindi-Reptilians during a boarding mission to a Xindi-Reptilian warship aimed at rescuing Ensign Hoshi Sato, who had been kidnapped by the Reptilians. (ENT: "Countdown")
In 2155 in a parallel universe, T'Pol used a stun grenade to incapacitate two MACOs who were guarding Captain Forrest, who had been imprisoned by Commander Jonathan Archer, and subsequently break him out of the brig. (ENT: "In a Mirror, Darkly")