WowAce

Keeps track of HoTs, DoTs, Buffs and Cooldowns. With audio and text alerts to refresh them.

Queing of /who and SendWho() queries and a much better interface (see docs), with gurantee to be executed & callback.

One large inventory.

Shows simple Buttons for your Cooldowns :)

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

Cryolysis 2

A collection of Rogue modules.

Henge

Shows skills in a tooltip and allows them to be used.

FuBar_HonorFu

Show item durability and quality in the character frame.

Cartographer_Treasure

Regain that wasted space!

Switch macro depending on zones or BigWigs boss modules.

CandyBar style cooldown display

Plugs classical world style fonts into LibSharedMedia-3.0

Necronomicon2

Modular and lightweight party/raid frames.

Drop the cheapest thing you're carrying

Target Debuff monitor. Stacking aware, giving current and potential debuff status.