Collecting Holotags

 refers to an unmarked and repeatable quest in Fallout 3.

Becoming the Messenger
After completing the main story line quest The Waters of Life, you will gain access to The Citadel, the Brotherhood of Steel's main headquarters in the Capital Wasteland. Inside the A Ring area, you will find a woman named Scribe Jameson. She is the keeper of the scrolls at the Citadel, which means it's her responsibility to record the actions of the Brothers, including their last action of all, their deaths. After speaking to her, she will tell you to bring her any Brotherhood Holotag you may find on your journeys.

Delivering the Message
At this point you probably already have some Brotherhood Holotags, which can be found when searching the corpse of a fallen Brother. If you kept these Holotags, you can give them to Scribe Jameson, who will thank you for bringing her this sad news. If you want to find more Holotags, you can search the bodies of any dead Brotherhood members, who can be found around the Galaxy News Radio building, near The Washington Monument, The Citadel, inside the Arlington Library and other places.

For the more daring, you do have the option of killing sleeping Brothers by the 'Mister Sandman' method. This nets you power armour and firearms, along with the holotags. Be careful to save before each kill, though.

Rewards
Scribe Jameson will pay you 100 caps for every Holotag you bring her, and additionally she promises that from time to time she will be able to give you "something a bit more interesting," alluding to the Brotherhood's ongoing technological research. You can return to her as often as you wish.