WowAce

Reports in raid chat whenever a raid member "fails." /EnsidiaFails to configure.

Displays the remaing cooldown of the action you tried to use

Database of Diminishing return categories and general data

Automatically enables or disables combat logging based on location.

GrindStatus

Bulk mail sending made really easy

Quest History tracker and shows lists of quests in each category

Checks your gear before bosses and yells at you if you're an idiot

Bulk mail reception made really easy

Bayi's Extended Questlog

Shows you a much bigger icon selection frame

Provides access to all Portal and Teleport spells on right click of one button.

Adds World Quests to the quest list

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

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

XRS

A blizzard group finder replacement

Manufac

CandyBar style cooldown display

Smooth combat HUD based on ArcHUD2