Dustman: Bag addon to auto sell particular items in your inventory when you reach any sellable area. Very customizeable too
Lore Books: puts all Lorebooks on your map
Minimap by Fyrakin: makes a minimap. a straight clone of the regular map, but mini! and on your screen all the time! Ive found this useful maybe 30% of the time. I still tend to use the big map
Recount: want to know if youre any good, or if one build is better than another, this is how! DPS tracking mostly
SkyShards: Puts all skyshard locations on the map
Slightly Improved Experience Bar (buggy at Veteran level): adds numerical values of your experience progress on the bar as well as keeping the bar active all the time. Veteran level has issues because of the Champion System, and thats relatively new.
Undiscovered: Shows quest hubs and other discoverables on the map. Very useful when you feel like youve completed a map, chances are, you havent!