Brotherhood of Steel holotag

Brotherhood of Steel holotags are a miscellaneous item in Fallout 3 and Fallout: New Vegas.

Characteristics
Military identification tags used by each member of the Brotherhood of Steel are a design dating back to the pre-War United States Armed Forces. They consist of two rectangular metal tags with rounded edges, worn on a ball chain around the neck, with a rubber covering on the edges. Personal information is printed on the front of each tag, with a fine print on the back and a holographic image embedded in the tag itself, to facilitate casualty identification for medical and burial purposes.

Fallout 3

 * Citadel: Killing non-essential inhabitants of the stronghold is a good source of tags, especially with Broken Steel installed. Hostility is tracked on a per-cell basis: killing characters in the courtyard does not turn anyone in separate cells hostile.
 * A-Ring: in the Citadel, has a holotag lying in the Den, on the small desk inside the cafeteria.
 * Citadel Exterior: respawning generic Paladins and Knights on the watchposts along the rim of the Citadel facades are a good source of both tags and power armor, particularly if they're killed by super mutants attacking the south side. In case they are unreachable, try exploding or tearing off their limbs with weapons or explosives.
 * Taft Tunnels, on the Citadel end is a living Brotherhood Paladin.
 * The Mall: 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 Brotherhood members 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.
 * Paladin Hoss is not essential and can be killed. He is necessary to complete a minor side-quest in Falls Church.
 * 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.
 * GNR building plaza is the site of 2 fights between several Lyons' 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
Holotags can be found on every member of Brotherhood of Steel (Mojave chapter), but are not used in any quests, remaining a pure flavor item.

Related quests
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.