Just like the Mass Effect wiki, there is a parallel for Dragon Age games called the Dragon Age wiki. It has been the best resource so far with information for almost everything in the game you would want to know about. Another good resource is GameBanshee’s Dragon Age 2 subsite. It’s not quite as up to date as the wiki; single author guides can never keep up with a wiki, but it does have fewer ads and a better layout for some things.
Warning: There are spoilers on this page.
I. List of Merchants
When you have sovereigns to spend, the wiki has a nice list of all the merchants. It’s nice to take visit all of them and see if they have any good items.