This is going to be short and sweet. No sprawling tale of where to farm and at what times to post in AH or what rare spawn to camp and weapon/armor to PRAY will drop...

...LIGHT FEATHERS. Yes, light feathers. Those things that fall in abundance from the moonkins in Darkshore and the birdmen in endgame. Harpies are particularly generous as they drop not only the feathers, but VIBRANT FEATHERS as well which you can exchange at the Darkmoon Faire for some pretty sweet prizes (like a 16-slot bag--40 req'd, I think).

My proof? To get funding for the guild I created to have more than the obligatory singluar bank tab, I began selling a lot of the stuff I had compiled in my bank on the six toons I had. Stuff I didn't know there was any practical use for. I sold a stack of 20 light feathers at the AH for 5g. I never though they'd really sell, but I was the only one posting a full stack, so I figured what the hey? When I found out how lucrative this was, I began buying out all the small stacks and (and later on, full stacks) of light feathers anywhere from a few silver for one or two feathers, to 2-5g for a full stack.

I know what you're thinking: "5g for a FULL stack? That's how much you just sold one for!" True, but by now I was selling them anywhere from 7-9g (10g was the most I ever got for a stack, but that was a rarity). And if you're thinking buying out all the light feathers is foolish because it'll leave you with a heavy inventory (c'mon, this is basic economics 101), don't worry. If you ride it out, much like the stock market you are guaranteed to make your money. With a post fee of LESS THAN 1 SILVER for a 48 DAY listing, you will surely make the money back even if you sell an originally 5g stack for 6...the profit margin is there. Mostly you'll see stacks for around 2g, but you'll come up if you buy the little half-stacks or cheap solitary feathers unknowing players list in AH.

Hope this helps get you to your next bank tab, mount, or that coveted epic you've been trying to get!!!