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.

you may also just give the scrap metal for free to walter for positive karma this is good if you are low on karma because the scrap metal is simple to obtain.

Winthrop in Underworld
Winthrop in Underworld also trades for scrap metal. He will give Stimpaks or Rad Aways in return for 5 units.

(Unless you think you're making a difference, selling scrap here is not as profitable as other scrap buyers. For instance, selling to Walter in Megaton will get you ten caps each; Stimpaks cost generally 30 caps. Also Casdin will give you 1 stimpack for 2 scrap metal. By selling in underworld you lose profit if you were to trade the scrap metals for a Stimpak.)

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

Ammo Press at The Pitt
The Ammo Press in The Pitt is located in the Steel Mill. Scrap metal is very useful in the presses due to it yielding more ammo than scrap put in. For example, if one puts 500 pieces of scrap metal into the press to make shotgun shells, it will return 1500 shells.