WowAce

Allows you to prevent addons from sending and receiving messages.

Draws some lovely 3D portraits in the standard Blizzard artwork.

Animated target indicator

Announces interrupts in the chat

OneBag

Client for MagicDKP which handles standby calls and bidding on items.

Cancellation

RBM

Keeps track of gold and cash flow across multiple characters

A superior broker/LDB display for users of the Recount damage-meter.

Considers an item against all the items you have to determine whether or not it is an upgrade

Mapster's Minimap brother

Add lines to ore and gem tooltips that show prospecting sources/results.

Shows where the equipment of inspected users or yourself is from.

This will keep track of how much you have paid per item in your inventory. Only one of (Auctionator, Auctioneer, AuctionLite,...

Allows for easy congratulating when someone earns an achievement.

Tracking and ranking of dps/hps in a guild.

Guilder brings common guild-raid tasks together in one convenient place.

Monk Brewmaster/WindWalker Priority and Buff/Debuff Tracking Tool

Battleground Timers in a HUD around the Blizzard BG status