Fallout exploits

Exploits are "holes" in the game programming which allow a player to do something which the developers did not intend and which alter the balance of the game in a way that benefits the player.

Vault 13
Infinite XP received for completing the Find the Water Chip quest repeatedly. Talk to the Water Merchants or keep the chip, travel to Necropolis and use it on the water computer there, resetting the water chip quest. Return the Water chip to the Overseer in Vault 13 again for 7500 more XP. Each trip between between the Hub and Vault 13 earns 8500 XP (and increases the time limit).

Shady Sands
Infinite Antidotes (each one takes four hours of Fallout-universe time): With only one Radscorpion tail in inventory, speak to Shady Sands' radscorpion expert, Razlo, about it. When he replies, Barter the scorpion tail to him. He will give the usual Antidote anyway. Antidotes sell for five times as much as tails, and nearly five times as much as it costs to buy the tails back. Note that he takes four hours to make them, and is only open from sometime after 6 AM to 7PM.

Raiders
Infinite XP: The lower, jail door will be opened, should the Vault Dweller have negotiated for Tandi's release. This open jail door can be 'unlocked' repeatedly for 25 XP each time (it may have to be closed and then re-opened first) Threatening Garl with the retribution of forces beyond his comprehension does not make the gang hostile until the Vault Dweller leaves the map and the XP for releasing Tandi is received. There is a lockpicking set in the refrigerator.

Junktown
Infinite XP: If you kill Neil with the Skulz gang and leave to the south and save, every time you enter and reenter the Junktown Zone with the Skum Pitt will display "you busted a cap in Neil's Ass" and grant you 300 XP.

Brotherhood of Steel
Infinite money: On level four, speak to John Maxson; offer to scout the area north of the Brotherhood base for mutants. He will give 500 bottle caps, or 1000 with a Barter check, repeatedly

Extra point of Intelligence: On level two, speak to Dr. Lorri, who can increase the SPECIAL stats other than Charisma and Luck, for thousands of caps and weeks of surgery each. A different scenario plays out for stupid characters; the logic of the code is that because raising Intelligence might make stupid characters normal again, the normal surgery scenario must not be available afterwards. However, this does not prevent a stupid Vault Dweller from taking Mentats, receiving the normal surgery first, and then receiving the stupid surgery. Nor does it prevent a normal character from receiving both surgeries, by taking lots of Psycho, waiting for Mentat withdrawal, or its addiction for the second surgery.

Boneyard
Infinite Deathclaw spawns: More legitimate than the other exploits, as it was obviously intended that the Deathclaws respawn until the mother and eggs below were destroyed; however, killing the Deathclaws repeatedly defeats the object of travelling and facing random enemies

Extra point of Luck: Speak to Chuck in the tent that is furthest east. Choose the provocative-sounding "Why do you assume that I need something?", accept advice, then thank him. Speak to him again three more times, each time asking for advice and thanking him. The Vault Dweller now gains a Luck point; because the Vault Dweller has not yet asked him what he does, though, the whole dialogue tree, complete with free Luck point, is still available

Infinite stimpaks and caps: After completing the quest to free Adytum from the Regulators given by Razor, there is a man near the Nuka-Cola man that gives supplies, if he is told to "keep the supplies", he will give 4 stimpaks and around 25 caps, this can be done infinitely.

Infinite caps: Save your game. Steal from Smitty for around 500-700 caps and then save your game again. Load, and Smitty will have more caps to steal. This can be done as many times as you want.