Some easy ones I'll add are old level 60 instances, lazy people after the ambassador title and or tailoring pay through the nose for runecloth, others are the same with the old enchants from there you can get from sharding things such as crusader.
Chunks of boar meat: Again due to lazy people these can fetch a tidy profit rapidly on some realms by those who want to power level cooking. You can get these from any low level boars, including the ones that wander around right outside some big cities. (Handy for alts on new realms)
Outlands heroics and associated dailies: These net nice amounts of vendor / quest gold and you can turn sharded items in to lolgoose enchant scrolls, etc. Netherweave cloth when turned in to heavy netherweave bandages also vendors for 6g a stack.
Scholazar basin + gathering profs and fishing is win.
Install the npcscan and npcscan overlay addons and hunt the northrend and outland rares if you are on at obscure hours, the gold dropped by northrend mobs alone is usually 25g+ along with cloth and any crystalized elements.
Farming the blue dragons in azshara with a skinning char for an hour or so can be beneficial as the leather from those can still fetch a reasonable amount of gold per stack and theres a chance of a rare whelp pet dropping, which is an instant sell for several hundred gold on the ah if it drops.
Excess Frostweave Cloth: If you have a tailor, use it to make frostweave bags and auction them all off, people will have your hand off for these every time.
Tyrs hand in eastern plaguelands is a good farming spot and the crusader enchant formula drops from the mobs here and ti auctions for a couple of hundred gold usually, though its a bit luck based. (The archmages seem to drop it the most)
Check for teremus the devourer each time you go through the blasted lands portal, he is a free 100g or so if killed once you turn in any orbs he drops to the broken npc near the road.