This probably isn't that useful since people seem to be in a hurry to jump to Northrend as soon as possible, but here it is anyway.
At level 67, you can do a quest chain ([Teron Gorefiend, I am...]
) that rewards one of several helms that, when equipped, allow you to see see some invisible mobs around the base of the Black Temple area in Shadowmoon Valley, near the road.
This is the plate helm... [Overlord's Helmet of Second Sight]
. When I was level 70, I think I took the spellpower mail version because it was more useful to stack spell power for AoE grinding.
Anyway, these invisible mobs drop a decent amount of money, cloth, and various greens. The respawn rate is pretty good. Plus since they're invisible to anyone not wearing the helm, you never have any competition.