WowAce

Displays Pet Info for Hunters and Warlocks. Also keeps track of Stables and Pet and Demon skills.

FuBar_DuraTek

Displays the locations of the chests for the Treasure, Treasure Everywhere achievement

Adirelle's unit frames, based on haste's oUF.

Shows you a much bigger icon selection frame

BankAccountant

WhoDrop

One large inventory.

Keeps account of attendance and loot for raids, parties, bgs, arenas and solo for questing.

ColaLight

MorgDKP

Keeps track of quests.

Death analysis tool

Modular and lightweight party/raid frames.

Switch macro depending on zones or BigWigs boss modules.

Manufac

ArcHUD2

FuBar_ItemDBFu

Try on different gear and see their total stats and their differences. In short, an ingame CT-Profiles-like addon.

Questing minion