Which Fallout 3 items are sought-after by NPCs?

Question

Some of the items in Fallout 3 are special because some NPC wants to collect them. What are they? Who wants them? What are the special conditions associated with turning them in to the NPCs, if applicable?

If it's not already clear, the answer(s) will contain spoilers.

Answer 1

1. Blood Packs - For the unmarked repeatable quest I Want to Drink Your Blood, available after completing the quest Blood Ties from Vance in Meresti Station

2. Cameras - For the unmarked repeatable quest Getting Ready for Prime Time in the Broken Steel DLC from Scribe Rothchild in The Citadel

3. Fire Ant Nectar - For the unmarked repeatable quest Nectar Collecting for Fun and Profit, available after completing the quest Those! from Doctor Lesko in Marigold Metro Station

4. Brotherhood of Steel Holotags - For the unmarked repeatable quest Collecting Holotags from Scribe Jameson in The Citadel, only availble until you start the quest Take it Back!, unless you have the Broken Steel DLC, which allows you to to continue after that quest.

5. Nuka-Cola Quantum - For The Nuka-Cola Challenge sidequest from Sierra Petrovita in Girdershade (Ronald Laren wants them too.)

6. Pre-War Books - For the unmarked repeatable quest Yearning for Learning from Scribe Yearling in Arlington_Library

7. Scrap Metal - For the unmarked repeatable quest Walter's Scrap Metal from Walter in Megaton, availble after the umarked quest Treatment

8. Sensor Modules - For the unmarked repeatable quest Getting Ready for Prime Time in the Broken Steel DLC from Scribe Rothchild in The Citadel

9. Sheet Music Books - Only during the quest Agatha's Song from Agatha in Agatha's House

10. Sugar Bombs - For the unmarked repeatable quest Murphy's Bombing Run from Murphy in Northwest Seneca Station

Naturally, killing the NPC will make you unable to deliver anymore items to that NPC

Source: The Vault

Answer 2

Scrap Metal

• Walter, water filtration tech
• Megaton
• 10 caps/piece
• Available after fixing the three leaky pipes around Megaton

Scrap Metal 2

• This Old House
• Winthrop in the Underworld at the Museum of History
• 5 pieces of scrap metal to exchange for 1 stimpak, 1 rad away, or 1 rad-X

Pre-War Books

• Yearling, scribe
• Arlington Library
• 100 caps/piece
• Available after reaching the library

Sugar Bombs

• Murphy, chem cooker
• Northwest Seneca Metro
• 15 caps/piece normally, 30 after passing a speech check; also access to Ultrajet
• Available after reaching the Metro station

Cameras

• Rothchild, scribe
• Citadel Laboratory
• 100 caps/piece and 25 exp/piece
• Available after completing Who Dares Wins, the last Broken Steel quest

Various

• The Outcast Collection Agent
• Protector Casdin at Fort Independence
• Scrap Metal
• Various weapons and armour
• Produces:
  • 5.56mm ammunition
  • Frag grenades
  • Rad away
  • Stimpaks

Answer 3

Walter in Megaton collects scrap for caps.

Vance in Arefu collects blood packs for caps.

Sierra Petrovita collects Nuka Cola Quantums for a quest, a recipe, and eventually caps.

Most of these repeatable quests are listed here, near the edge of the page on the fallout wiki.

Fallout 3 is a massive game. This question needs to be tightened in scope, a little. It's too broad.