MarketWatcher is an addon that lets you track specific items and draws a graph for it.
This can be very helpfull for items you trade a lot or items with an unstable price.
You'll need to scan with it apart from the normal AHscan. and after 5 scans you'll get the graph.
This let's you follow the minimum, medium and maximum price from the auction over a period of a month. I use this to see if the current price is low compared to other dates. You'll probably also see a trend, That in the weekend the price is lower/higher then in the week. So you'll know when to buy and sell. This is extremely good if your new to the game or to a market you want a piece from.
the Addon can be downloaded at Curse
A fortune starts with one copper
(yes i'm addicted to that sentence ^^ )