Lightweight, unobtrusive addons:
I have always used Mobinfo2, when it wasn't broken. The latest incarnation is Mobinfo3, which will hopefully live up to its predecessor. Unfortunately, at the moment, you need to also install ItemDataCache for it to work.
This handy little addon simply adds additional information to the tooltip that appears when you mouse over something, such as an item or a corpse. It will tell you which mob(s) dropped the item, the drop percentage, vendor price, how many times you've killed that mob, information about the mob's stats, and more. It is based on your own work, so the drop information is not "the" drop information, but rather the statistics on how it has dropped for you.
While this may not be the best solution at the moment, it is something to keep in mind for the future.
If you want to keep it simple, there are two addons that will do the trick. SellFish will show you the vendor price for an item in its tooltip, and Vendorbait is a mod specifically designed to work with it. When given a choice of quest rewards, it will highlight the one that sells for the most so that you can decide at a glance.
ItemPriceTooltip is another simple, low-memory addon that will publish the vendor price of an item in the tooltip.
Heavier, more inclusive addons:
Auctioneer and WoWEcon will both give you vendor information, along with auction prices and other financial goodies you might want to know. Both use a lot of memory, and both require that you spend a bit of time syncing your addon with the server at each login. Auctioneer takes up the most time, while WoWEcon is less laborious.
Auctioneer provides you with auction information based on what items are being listed for, while WoWEcon scans what price items have actually sold for.
Choosing between the two will depend highly on personal preference. If you like to try to move items at a high price when they are in high demand, and all of the cheaper, or more "reasonably" priced items have sold, then Auctioneer might be your best bet, as it allows you to monitor the listings.
If you'd rather be given solid prices that will result in more consistent sales, then WoWEcon might be better for you.
Of course, if you really don't spend much time auctioning, then I don't recommend taking up the memory with one of these addons. Your game will run more smoothly without them.
In a future article, I will discuss how addons like these can enhance your ability to auction effectively.
Other vendoring addons:
VendorFood is a cute little addon that won't take up much room and makes vendoring your items a breeze. Once it is installed, just head over to a vendor. In the normal window that comes up, you'll notice a button that says "Feed Vendor" as pictured in the main image for this article. Clicking it will automatically sell every grey item and tally up how much money you made doing it! This is an "I was totally born yesterday" addon. No work, no fuss, completely fool-proof vendoring.
If you ever used AutoProfit, this is very similar, although currently less customizable. SellJunk is another option, and it will allow you to add other items that you would like to automatically sell, such as Fish Oil, that is technically a white quality item.