Quest Reward Collectors

In Nightfall, you can receive special trophy items as quests rewards. You can turn in these trophy items to special Collectors; they can exchange them for items that aren't available from normal Collectors. Like other Collectors, these NPCs appear with the word [Collector] after their names.

The trophy items you can receive for completing quests vary depending on the province or realm where you started that quest.
In Istan, you can trade in Battle Commendations to Sunspear Quartermasters.
In Kourna, you can trade in Kournan Coins to Smugglers.
Some Vabbian quests reward you with Trade Contracts to Vabbi Trade Officials.
Adventurers in the Desolation can obtain Ancient Artifacts and trade them to Wandering Scribes.
In the Realm of Torment, you can trade in dark and mysterious Secrets to Secret Keepers. http://www.guildwars.com/gameplay/gwnfeatures/collectors.php