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.
Best Answer
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
Cameras - For the unmarked repeatable quest Getting Ready for Prime Time in the Broken Steel DLC from Scribe Rothchild in The Citadel
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
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.
Nuka-Cola Quantum - For The Nuka-Cola Challenge sidequest from Sierra Petrovita in Girdershade (Ronald Laren wants them too.)
Pre-War Books - For the unmarked repeatable quest Yearning for Learning from Scribe Yearling in Arlington_Library
Scrap Metal - For the unmarked repeatable quest Walter's Scrap Metal from Walter in Megaton, availble after the umarked quest Treatment
Sensor Modules - For the unmarked repeatable quest Getting Ready for Prime Time in the Broken Steel DLC from Scribe Rothchild in The Citadel
Sheet Music Books - Only during the quest Agatha's Song from Agatha in Agatha's House
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