Planting items

 or reverse-pickpocketing is the technique to give items to NPCs through the usual pickpocket interface. It's done by picking the pocket of an NPC, then proceeding to place an item on him/her/it, rather than stealing one. The transaction can be detected and blocked as if you would steal something.

Fallout and Fallout 2
Item planting can play a minor role in the game, most apparent in Adytum in Fallout. By planting items, you can equip the Adytowners (or any other non-hostile NPC) with weapons to revolt against the Regulators. This is limited by the engine of the game, as certain NPCs are unable to use certain types of weapons, due to missing animation frames. This method can work in any place with non-hostile critters, but its usefulness is limited by the number of occasions in which you can equip NPCs to fight other NPCs.

Of course, this can be used in conjunction with TNT to cause an NPC to explode after a predetermined amount of time, via the timer you set. Be warned, surrounding NPCs will know you are responsible, even though they have no way to know. Additionally, if the first explosion did not kill the NPC outright, it will turn hostile. Another danger is that killing an NPC by planting a bomb in his/her inventory causes the game to crash a majority of the time (allegedly, only a few can be killed without crashing, though they will STILL crash the game both before and after patching)

This is the only way to kill an NPC using explosives, as simply dropping an armed bomb to the ground will cause hostility. If a bomb cannot be planted without getting caught, you might try planting it on another NPC (as an unwilling suicide bomber) or on a container (just make sure the first blast is lethal or that the NPC is standing in front of a wall).

A word of warning: if you manage to plant a bomb on an NPC, don't save the game before it goes off! The file will be corrupted, resulting in you unable to load it.

Fallout 3
In Fallout 3, every transaction is checked and results in a loss of karma (with the exception of evil NPCs), regardless of whether you're stealing or placing an item onto the NPC. It can also be used to obtain equipment from characters that you would otherwise be unable to steal, because they currently have it equipped.

Planting for Exploding Pants
It is possible to place an explosive device, such as a Frag Grenade, Frag Mine, or Plasma or Pulse variant upon an NPC by passing one from your inventory to the NPC's inventory via pickpocketing. It will be labelled as 'Live' and will be unusable in the target's inventory thereafter. After the inventory has been closed, the target will realize their fate and run, before exploding after about 3 seconds. This will kill the NPC, regardless of health or the player's explosives skill.

This should be done at maximum pickpocketing distance to allow the player distance to run from the blast radius, or by using a pulse explosive which does minimal damage to the player. It should be noted that if the target is carrying the same type of explosive as the player is trying to plant, the player will always be caught.

The famous adventurer side-kick Argyle has dubbed this maneuver the Shady Sands Shuffle. Each kill done using this method will be recorded as a Pants Exploded, and will also earn you the Achievement/Trophy Psychotic Prankster.

Planting items to obtain gear
Some very powerful and unique gear is available (almost) exclusively through exploiting this method - items normally worn by quest NPCs that they never take off, or cannot be killed for. To get someone to switch out their armor, you must first place an item that occupies the same slot onto the NPC (e.g. Metal Helmet to obtain hat and glasses, or Raider Blastmaster Helmet to obtain just the hat, not glasses). Upon the next cell reload, it is advised to leave the area or enter another building then wait for a few hours, they will equip the replacement item, putting the target item into their inventory. It can now be pickpocketed as normal. (Note: An easier way if you have all the autosave options on is to leave the cell (area) forcing an autosave, immediately load that autosave and turn right back around and re-enter the original cell (area). This ensures that the character pickpocketed will be in the same spot as when you last saw them.)

For armor, the replacement item needs to have a higher DR value than the target item to have the AI equip it. That means that you can't get everyone to wear Sexy Sleepwear, since it doesn't give a DR increase. (PC users have console commands to get around this limitation. See Mesmetron for non-quest NPC alternatives.)

For weapons, the DMG/DPR needs to be higher, although this is volatile and may vary from NPC to NPC. In some instances, the weapon is still unavailable to you. Use for this purpose with caution.

The NPC will switch armor/weapons if you leave the area for a long time, or leave the area and do a save followed by a load and return to the area with the NPC again. In most instances, the simple autosave suffices.

By using a Radiation Suit, it is possible to make NPCs put face- and headgear in their inventory without changing their worn armor. This works even if the worn armor has a higher DR than the Radiation Suit, and seems to work most of the time if the DR is lower than the one of the Radiation Suit, though not always. More testing is required. This trick also works on characters such as Three Dog or Lucas Simms, letting you steal their unique headgear without having to give them another piece of equipment or making them wear other armor. The player can then pickpocket the Radiation Suit back for no Karma loss and make the NPC wear their original headgear again, should he or she wish to.

Examples
Items that are not available through other means:
 * Elder Lyons' Robe
 * Dad's Wasteland Outfit
 * Kid's Cave Rat Outfit