If you want to focus on making money, there are several plugins that may prove useful, and the following are especially worth considering.
For gold-conscious players, the auctioneer is an important tool to ensure you maximize your earnings. Not only does it simplify the experience of the auction house itself, let you easily cut existing listings, buy at the cheapest price, and track the market, it also shows the auction price of the items in your bag, allowing you to judge Value, you pick up something while in the outside world.
If you have a gathering profession, a plugin like GatherMate may make it easier for you to ensure that you don’t miss a herb or a mining node. GatherMate will remember the location of each herb and ore you collect and display this data on mini maps and world maps to make collection easier and more efficient.
For those who really want to master the money-making skills in World of Warcraft, TradeSkillMaster is the ultimate plug-in, a powerful set of tools that covers everything from handmade to auction. However, at the complex end of the World of Warcraft plugin, it definitely needs some learning for beginners, so unless you really want to collect every copper you can collect, it may not be worth studying in-depth.