Auction & Economy
Auction House
Auctionator (Zirco) – Auctionator is an easy-to-use World of Warcraft add-on that helps you buy, sell and manage auctions. The most complete and up-to-date information can be found atauctionator-addon.com.
Auctioneer Suite (Nechckn) – Auctioneer is a very popular auction addon mainly used to price items easier, faster, as well as appraisals, enchanting, disenchanting, tracking prices of things, finding certain items by buyout, bid, if cheaper than vendor price, etc.
AuctionLite (Merial) – AuctionLite is a simple, lightweight addon that tracks auction house data and improves the interface for buying and selling items. AuctionLite allows you to: Scan the auction house, Search for and buy out auctions, Create auctions based on the current competition, Display tooltips with auction and vendor data.
AuctionMaster (Udorn) – AuctionMaster is a compact auction house tool for being able to sell items like the wind, scanning auctions for item statistics, searching for wanted items in the auction house, looking for good opportunities among the current auctions and displaying the statistics gathered so far in the tooltip of any item.
Auction Profit (milkmoocow) – It basically adds up all your auction’s bids and buyouts if they are all sold in a little frame at the bottom of the auction tab.
MarketWatcher (Torrid01) – Market Watcher is an addon that scans the auction house and records data for specified items and assists in daily auction house shopping in general. When the addon is set to record, a price history can be displayed to assist in determining a value for items of interest.
QuickAuctions 2 (Shadowed) Quick Auctions is for people who want to automate auction posting and cancelling and don’t want to deal with checking 50 auctions, seeing if they were undercut, cancelling and repeating. While this is primarily meant for automating large quantities of auctions it works fine for a few auctions too. This does require a little bit of tweaking for yourself, but the latest version provides a GUI configuration and is much easier to setup and manage. Also includes a summary window that lets you quickly see the status of a group of items on the auction house and queue items for the craft queue to make it easier to see the status of an entire segment of the Auction House market.
Economy

Analyst
Analyst (Bethink) – Analyst collects information about the virtual economy and presents that information to the player.

Gnomish Yellow Pages
Gnomish Yellow Pages (lilsparky) – The Gnomish Yellow Pages will monitor all chat traffic to identify people advertising trade skills. As trade skill ads are identified, they are recorded and stored for later retrieval.
Sell-O-Matic (Silmano) – The purpose of this mod is to help us sell the kind of items that we get most frequently: For example, you’re fishing and loaded with raw fish or gray items.

UnitPrice
UnitPrice (xbeeps) – Adds the current bid and buyout price of a single unit below the name of the item in the Auction House. Makes it easy to get an overview of price ranges and to tell the good deals apart from the bad deals.

VendorSearch
VendorSearch (adriweb) – Creates a search bar to search through a vendor’s items. You are now able to find much faster the item you want.