Cannibal/Fallout 4

Cannibal is a perk that can be utilized in Fallout 3, Fallout: New Vegas, and Fallout 4.

Fallout 3
With the Cannibal perk, you are able to feast on dead humans and non-feral ghouls by being in sneak mode and activating the "Devour" option. The feast will restore 25 hit points, but add 2 rads and cause you to lose one point of Karma. If anyone sees you devour a human, it is considered a crime against nature, turning all friendly non-player character's in the area hostile.

It also expands dialogue options with The Family in the Blood Ties quest and when speaking with Jack Smith in Andale (without the perk, this option requires a speech check).

Fallout: New Vegas
The perk functions the same as it did in Fallout 3, except that it can be acquired much earlier (at level 4 instead of level 12). Should Hardcore mode be enabled, corpses will also reduce the starvation meter by 25. After consuming 25 corpses, the player will acquire the Dine and Dash challenge perk, allowing them to harvest human remains, rather than eat the corpse directly.

Cannibal also provides additional dialogue options; in the Beyond the Beef quest, when talking to Marjorie, there's a dialogue option that says, " It's okay, you can be honest with me. I eat people too, you know". With this perk, the 'lie' part is erased. Mortimer also has some dialogue options. In addition, in Dead Money, you can ask Dog how the ghost people taste.

Fallout 4
With the first rank of this perk, feasting on human corpses restores a small amount of health. The second rank allows one to eat ghouls and super mutants to regain lost health as well. The third rank significantly increases the amount of health restored whenever eating human, ghoul or super mutant corpses. Cannibalizing corpses in Survival difficulty will lead to the dark craving effect.