Brotherhood of Steel holotag (Fallout 3)

Every member of the Brotherhood of Steel, just like the soldiers of the US Army that they are descended from, wears a set of two Holotags around their neck. They contain essential personal and medical information about the wearer so that their corpse can be identified.

Unmarked quest: Collecting Holotags
In Fallout 3, holotags recovered from fallen Brotherhood members can be turned in to Scribe Jameson at the Citadel (A-Ring Archives) in exchange for 100 caps and 25 XP each, but only before embarking on the Take it Back! quest. With Broken Steel, this can be continued beyond the Take it Back! quest. Jameson also talks about offering technology from time to time in return but never gives any.

Locations

 * More ruthless players may consider slaughtering the various inhabitants of the Citadel. This can net you dozens of Holotags, particularly once you have downloaded Broken Steel. Scribe Jameson, along with the rest of the faction in A-Ring, will NOT become hostile if you kill their comrades in different sections (e.g. Courtyard, Lab, B-Ring if she isn't in there).
 * A-Ring, in the Citadel, has one in the Den on the small desk just before the room with the beds.
 * Taft Tunnels, on the Citadel end is a living Brotherhood Paladin.
 * Citadel Exterior, the unnamed brotherhood members high on the catwalks using the sniper nests (due south of the citadel gates) may be killed by the player as well (the unnamed member outside near the sentry bot only gets knocked unconscious). They are not guaranteed to always fall with access to their inventories, use a scoped weapon to zoom in and explode their corpses to gain access depending on gore dispersal. They may also be killed by super mutant overlords (if Broken Steel is installed ) or lesser super mutants without Broken Steel, allowing the player to easily miss them as the battle occurs south of the Citadel entrance and is activated whenever the player arrives at the Citadel despite not being within earshot of the firefight.
 * Note: The sniper nest/catwalk brotherhood members will respawn when the super mutants do making them an unlimited source of the tags and Brotherhood power armor.
 * The Mall has three: 1 in the bunker, 1 bent over a sandbag wall outside one entrance to the bunker, and 1 bent over a raised metal redoubt on the edge of The Mall. More either respawn or are found in already dead in the trenches near the bunker entrance.
 * Falls Church is another location that usually has one or two. Upon reaching the location, there will be a big fight going on between Brotherhood of Steel members and super mutants. They will not be able to kill all of the enemies. Every corpse from a Brotherhood member will have a Holotag.
 * Note: try to keep Paladin Hoss alive. His survival is necessary to complete a minor side-quest in Falls Church. Initiate Pek does not seem to carry one.
 * Olney Sewers, there is a holotag on the dead Brotherhood Initiate with the prototype medic power armor.
 * Arlington Library - several Paladins will attempt to clear the library of raiders when the player arrives. If any fall, their tags can be collected.
 * Galaxy News Radio is the site of 2 fights between several Brotherhood of Steel soldiers and super mutants. A few soldiers will likely fall in the battle.
 * Jefferson Memorial is often a site of fighting between Brotherhood of Steel soldiers and an Enclave Sentry Bot and an Enclave soldier if Broken Steel is installed. A few soldiers will probably die by the time they eliminate the threat, allowing you to take the dead soldiers' Holotags.

Fallout: New Vegas
In Fallout: New Vegas, holotags are dropped by dead Brotherhood soldiers but are useless because they are not part of a quest (unlike in Fallout 3). This can be found on every member of Mojave Wasteland Brotherhood of Steel.