WowAce

Tracks rare drops, including companions and mounts

Death analysis tool

Provides the /in command for delayed execution.

Attach diminishing return icons to unit frames.

Tells you if you're buffed

Show buffs on nameplates.

Don't accidentally solo queue BGs when in a group!

Plays a sound when feasts are dropped, even when sound is off.

Shows you the radius of your totems.

Adds item values to chat messages.

Adds more powerful search features to AtlasLoot.

Displays the internal cooldown of Eclipse as a ClassTimer bar

GemCensus inspects people and records which gems they have socketed. Then displays which gems are the most popular.

Adds macrotext to action buttons without the need to create new macros for each button.

GBS tricks other addons into thinking a guild bank is your personal bank.