Auctioneer

If you only ever install one World of Warcraft add-on I recommend Auctioneer.

To quote from the documentation here:

Auctioneer is an addon that allows you to view information about an item that may be useful in helping you determine what it's worth. Straight out of the box, Auctioneer knows much about many items in the game, and will tell you many things about the item when you move your mouse over the item in the game.

You will make more WoW gold by installing and using this add-on than just about anything else.