The Slacker's Guide to World of Warcraft

Auctioneer
http://auctioneeraddon.com/

Adventuring in World of Warcraft is expensive... between buying gear and new skills (not to mention fixing the mess you've made of your talent tree), you need a constant influx of gold, and lots of it. Rather than be completely berated for trying to panhandle it out of the never ending stream of high level dudes running around town, make some yourself with the help of Auctioneer. This add-on will scan the Auction House and record every single thing in it (the more you scan the Auction House, the better cross-section of prices you'll get). It also includes functions that will alert you to when something is being sold too cheaply, enabling you to snipe the auction and charge a more appropriate, gold-sack-fattening price.