WowAce

Blocks Chinese Chat

Allows you to browse your clients database

Scorchio!

Displays the zone level ranage of the zone you are in and also gives you a suggest zone and instances to be in.

RapidCast

Keeps track of space left in your bags.

Tracks skill and tradeskill costs and progressions, as well as trainer locations.

World of Warcraft Classic addon to display current zone information.

TGuildFrame

Records mail when visiting the mailbox to make keeping track of mail and expiration dates easy.

Disease Tracker for Death Knights.

Attach diminishing return icons to unit frames.

MainAssist

Monk Brewmaster/WindWalker Priority and Buff/Debuff Tracking Tool

FuBar_uGuild

Provides the ability to suppress specific spellcasts like "Deaden" from your casting bars.

KoS-Management Tool

Stores a browsable database of spells that mobs cast

Keeps track of current money and all your characters on one realm.

Simplifies GDKP bidding processes