Planting items

 is the technique to give items to NPCs through the usual pickpocket interface. The trade is initialized just like normal pickpocketing, then proceeding to place an item on the NPC, rather than stealing. The transaction can also be detected and blocked as if you would steal something.

Fallout 2
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.

Fallout 3
In Fallout 3, every transaction is checked and gives karma loss (if any - evil NPCs don't), regardless which direction the transaction went. It can also be used to get equipment from characters that normally wouldn't be available.

Exploding Pants
Of particular note, the Pants Exploded statistic is derived by how many (live) grenades/mines are placed on an NPC. (The first grenade "planted" on an NPC is automatically primed.) This can be an effective stealth killing tactic, but be warned - if crouched, you may not be able to run away quickly enough and will be caught in the blast radius. Perform at max-pickpocket distance and immediately back away (or duck behind a nearby wall) for minimum damage to yourself. Frag mines seem to have a smaller blast radius, and the PC can easily get out of the blast radius while still crouching. Pulse grenades are also a good choice, as they will kill the target, while doing minimal damage to nearby organics.

Reverse-Pickpocketing 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, I advise you 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.

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
 * Dads Wasteland Outfit
 * Kid's Cave Rat Outfit