Earn Exalted status with the Argent Dawn.
This reputation grind has changed since Cataclysm. One needed an insignia to obtain Scourgestones and then turn in those in order to gain reputation.
The best way NOW to earn reputation is...to do the Eastern Plaguelands questline! (You were going for achievements anyways, weren't you? May as well knock those out.)
Along the way, you'll end up going to the Argent Dawn and starting the chain The Trek Continues
Upon completing said chain, you'll quickly reach revered with the Argent Dawn.
Reaching exalted from there is as easy as running Stratholme (I prefer side gate- the last boss can drop a mount.) and Scholomance
There are also repeatable quests. Annals of the silver hand
(hint: If you're impatient, clear the dungeon and leave but don't reset the instance. Turn in the quest, and then get it back- the dungeon will still be cleared. Although this is pretty pointless because the mobs die quick and you're missing out on potential greens. It's just worth mentioning is all.)
As well as Aberrations of bone
(Kill Rattlegore. You'll have to reset the instance in this one if you want to repeat it.)