Shadow Sword (Yu Yu Hakusho); possesses an antidote that reverts its Conversion Slash
Moon Pool (H2O: Just Add Water/Mako Mermaids); can turn merpeople back into humans and vice versa
Mystic Falls (The Vampire Diaries); temporarily surrounded by a magic-negating field that could revert vampires and hybrids to their original state
As the "lump" anti-elemental, Lumpy Space Princess (Adventure Time) was able to single-handedly undo the conversion effects of the four elementals.
After several failed attempts, Batman (The Batman vs Dracula) successfully created a cure for vampirism, allowing him to revert all the people Dracula converted back to their original selves with no memory of what occurred.
The Omnitrix (Ben 10: Alien Force) can repair genetic damage such as DNAlien mutation, restoring the victim's DNA to its original state.
After scanning the Omnitrix's ability to repair genetic damage, Cooper Daniels (Ben 10: Alien Force) was able to create rayguns...
...to accomplish the same effect.
The Moon Pool (H2O: Just Add Water) can turn merpeople back into humans, and vice versa.
While he can be temporarily turned into a vampire, Spider-Man's (Marvel Comics) radioactive blood and metabolism allow him to eventually recover from the effects and revert back to normal.
Anti-Venom (Marvel Comics) curing the Spider-Virus, returning the infected to normal.
With her yo-yo, Marinette Dupain-Cheng/Ladybug (Miraculous Ladybug) is able to cleanse akumas of their evil, undoing Hawk Moth's influence and turning supervillains back to normal.
The vampire cure (Supernatural) can revert vampires back to humans, provided the vampire has not yet fed on human blood.
The werewolf cure (Supernatural) can revert werewolves back to humans, provided the werewolf is in the early stages of lycanthropy and has not yet fed.
Retro-mutagen (Teenage Mutant Ninja Turtles) can restore mutants' DNA to its original state, curing them of their mutations.
Steve Rogers/Captain America's (Ultimate Marvel) immune system can fight off a vampire infection and revert back to normal thanks to the Super Soldier Serum.
In addition to being able to turn humans into demons by cutting them, the Shadow Sword (Yu Yu Hakusho) also possesses an antidote in its hilt that can revert the transformation.