These are the addons that I use in my UI that are related to auction house metagaming.

AddOn Control Panel - This useful addon allows you to toggle addons between on and off while logged on to your character, and also allows you to reload your UI. Very useful in some situations.

Altoholic - Altoholic can keep record of what items all of your alts have and can even show guild banks (for your bank guild, obviously). I use this when I am using Skillet to craft my glyphs to see which ones I have currently so that I always have 4 of each.

Auctionator - Now this addon is much better in my honest opinion. It has a much better selling UI, as you can put the item up and set how many stacks of X you want to post up, and it will undercut always. But the best part of Auctionator is the buying UI. You search for what you want, and it gives you the cheapest occurence of that item on a per-item basis. So a stack of 20 of X that is 10g will be priced lower than a stack of 5 of X at 5g. So it will enable you to always buy the cheapest items.

Auctioneer - Who doesn't use Auctioneer? It scans the AH and gives prices for many things and has a slightly better posting UI than the normal AH, and can sort things better when you search for them.

Auditor - Keeps track of all of your gold, incoming and outgoing, on many different criterias, like mail, vendors, AH, etc. You can also sort it by day, session, week, month, or total.

MySales - This addon keeps track of all of your sales and tells you who bought them. You can also add a POI (person of interest) and have it track them, so you can see who is buying up all of your stock all the time.

Postal - Auto collects your mail. That's all it does, besides tell you how much gold you collect, but extremely useful nonetheless.

Skillet - Extremely useful in that I can set up queues to craft items and the shopping list is saved between alts so I know what I need to buy and how much of it.

TradeSkill Master - I use TSM only for posting, because the group/post interface is the greatest thing ever. I do not use the crafting part, but you can find guides on how to use the rest of the addon on The Consortium Forums.