There are many addons that can fall in this category but, because I like to minimize overhead, I'm trying to avoid most of them. That said I still have a fairly long list.

Auctions: While I've had some success with WoWEcon the clear leader in this area is Auctioneer which has just been re-released for 2.0. Another interesting site is WOW Economy.

Inventory: ArcInventory not only combines all my bags in one, it also allows me to arrange the contents by type (and hide and show only certain types such as weapons, quest items etc.). The configuration is very easy (better than vBagnon I was using before). Another aspect is facilitating the swapping of gear for different situations. For this I use Outfitter that automatically switches to riding gear when mounted, AD Trinkets when in the Plaguelands etc.

Maps: It is almost impossible to get by without being able to plot Thottbot locations on the map. This can be done through big packages such as Cosmos or Titan Panel but also through my mod of choice, Cartographer (ace) that also provides me with instance maps as well as adds a bunch of information (all quest givers I've seen, vendors etc.). Finally it provides boss loot tables similar to AtlasLoot.

Items: You frequently see people asking you to "link item xyz". Saeris's LootLink provides a database of most items in the game (and all items you get to see yourself whether linked or otherwise are added). I found I seldom needed the full database so I'm currently using Fizzwidget's Linkerator.

Others: I use EzDismount to get me off my mount as I want to enter battle, jump on a griffon etc. For training pets, Fizzwidget Hunter's Helper is indispensable (when you target an animal it will show you which, if any, pet skills you can learn by taming that creature).