WowAce

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

Quest History tracker and shows lists of quests in each category

Drop the cheapest thing you're carrying

One large inventory.

Tracks the HoTs you cast.

Raid Frames using the oUF framework.

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

A blizzard group finder replacement

tracks and share quests between your party

Marks the locations of Midsummer Fire Festival bonfires on the worldmap and minimap.

XRS

Create panels for customizing your UI, sucessor to eePanels

Automatically enables or disables combat logging based on location.

Bulk mail sending made really easy

mount and companion manager

Bayi's Extended Questlog

Auto repair + shows durability.

Incubator

Reduces interface tedium by doing the little things for you.

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