I can't remember how or when I first stumbled upon SellJunk. Its page on Curse lists a creation date of November 17th, 2007 and boasts over two million downloads to date. It was probably sometime in 2008 or 2009 when I found it; perhaps a guildie tipped me off or I went searching out of need. Either way, SellJunk provides a valuable quality-of-life feature, even with the changes to bags and inventory space we'll see in Warlords.
As you probably guessed, the out-of-the-box addon streamlines the process of ridding your already-taxed-for-space bags of that annoying grey vendor trash. Simply interact with a vendor and you'll notice a button on their Buy/Sell screen conveniently labeled SellJunk. Click the button and the addon sells up to 12 grey items at once. Depending on how full your bags are you may need to click the button two or three times. There is also an option to instruct the addon to automatically sell all grey items upon opening the vendor window---no extra button clicks needed.
In addition to the convenience of easily selling grey items, there's also an exception list. To it you can add grey items you do not want to sell, i.e. you like collecting rocks; or conversely, add non-soulbound white/green items that you'd rather sell to the vendor than keep for whatever reason, i.e. low-level consumables, etc.
I can't guess how much time this addon has saved me over the years, but judging by my /played time it can't be an insignificant amount. It's especially powerful when questing/leveling or farming old content. Speaking of which, there's an extra feature for folks stuck with full bags out in the wilds, but don't have access to a vendor mount or pet: typing the command /selljunk destroy will destroy all of your grey items---as well as any other items you may have told the addon to target.
Convenience at its finest.
Is there an addon you know about that does what this one does, but better? Perhaps there's an addon I should know about. Let me know in the comments or shoot me an email.