Scrap Metal (Fallout 3)

Scrap Metal can be traded to various different NPCs located in Fallout 3.

Walter in Megaton
Scrap Metal is desired by Walter in Megaton after completing the unmarked quest: Treatment. He is located in the Water Purification shack during the day. He provides 10 caps per 1 unit of scrap metal and the Game provides 5 XP per 1 unit of scrap metal. For many players -- especially those who reside in Megaton -- Walter may be the most preferable option (even when XP is no longer an issue) as you can easily fast travel to Megaton to unload your stacks of scrap metal, and then head down the ramp to Craterside Supply to sell unneeded loot.

Note
When giving scrap metal to Walter you receive 5 XP per piece. If you have XP bonuses (from Well Rested and/or Swift Learner), you should note that the game rounds the bonus up. Thus, with a 10% XP bonus, you will receive 11 XP for all "pairs" of Scrap Metal (because (2 * 5) * 1.10 = 11), and 6 XP for an "odd" piece of Scrap Metal (5 * 1.10 = 6, since it rounds up).

Bugs
There is either some pathing issue, or perhaps a scripted event, that may eventually cause Walter to be found dead at the bottom of a slope to the left of the clinic. It's likely a pathing issue that causes him to fall to his death.

Winthrop in Underworld
Winthrop in Underworld also trades for scrap metal, but he will give Stimpaks or Rad Aways in return, per 5 units. Of the three options available for trading scrap metal, this is clearly the least appealing: Walter will pay 50 caps for the same amount of metal (also granting you XP when you do so), and Casdin will give you a Stimpak for only two pieces of scrap metal.

Protector Casdin at Fort Independence
Protector Casdin of the Brotherhood Outcasts is a much better option than Winthrop and is located in Fort Independence. He will trade 12 5.56mm rounds, 1 Stimpak, 1 Rad Away or 1 Frag Grenade per 2 units.