Grinding-  Grinding is where you find a certain spot of monsters and continually kills these monsters for a long period of time.  Grinding is one of the most boring ways of getting money but can be very profitable. 

Note: this guide was written before Burning Crusade was released. The tips are still generally good, bt yoiu will find some outdated sections, such as builds.

When I grind I make sure that I have food, and water if I'm a mana using class.  Then I'll bring a few potions incase someone attempts to gank me.  Also you can hire a friendly mage to make all this food and water but make sure to tip. 

Next consider this: is your current talent build the best for grinding? 

Say you're a protection warrior and you want to grind.  In one week of grinding you might make say 500-700 gold if you grind all day for 7 days.   Now consider this, as a fury warrior you could make up to 1000-1200 gold in that same 7 days.  Even if your re-spec cost is 50 gold you're still making more gold then it would cost you to re-spec.

• Warrior: Fury build 15 in arms 36 in fury
• Priest: Shadow build 20 in discipline 31 in Shadow
• Rogue: 20 in Assassination 31 in Combat
• Hunter: 31 in Beast mastery 20 in Survival or marksmanship.
• Druid: 20 in Restoration 31 in Feral Combat
• Paladin: 15 in Holy 5 in Protection 31 in Retribution
• Shaman: 30 Enhancement 21 in Restoration
• Warlock:  31 in Affliction 20 in Destruction
• Mage: 30 Fire 21 Arcane 
  

Now while your grinding look for eagle gear and of the Bear gear as you can sell these on the AH for a couple gold.  Another thing is the Major healing/Mana potions that drop off of high level monsters.  One Major Mana potion is around 1-3 gold each and a Major Healing is around 1-2 gold. 
 
Crafting:  
Arcanite Reapers:  These axes are very popular on newer servers as on newer servers there is less access to MC/epic weapons and thus the Arcanite Reaper is the best choice pre raid dungeon.  Normally the crafting cost for these without the buyer having the mats is around 500-800 gold.  If he has the mats then it is around 15-50 gold. 

Another thing, if you are one of the first players on your server to get the Arcanite Reaper plans then you could charge all the way up to 100 gold to craft it. 

Another great seller is the Lionheart Helmet this helmet similar to the Arcanite Reaper is very popular even on older servers.  Selling both of these items can make you thousands of gold. 

Instance runs:Another way to make money is to charge low lvls for running them through instances.  Say I run 4 people through The Deadmines for 1 gold per person that's 4 gold and depending on your class you could run the instance rather fast. 
Another thing is you can say is to tell them to let you have all the linen or silk or mageweave and then when your done with the instance sell all of it on the AH for a nice amount of money. 
Also for higher level instances like Scarlet Monastery you can run them through it for a couple gold each person and take the blues they don't need to disenchant and sell on the AH. 

Instance Farming
Rogues and druids are especially adept at this but any class can do it.  What you want to do is find the fastest way to either 1. Kill a large amount of elite monsters then reset the dungeon.  Or 2. Kill the boss and disenchant the drops to sell on the AH. 

For a Warrior or a Mage/Warlock you might want to do the first then the second so you would AoE the monsters to death then kill the boss disenchant all the blues you find and reset the dungeon. 

For melee classes you can kill Herod and get his weapon “The Ravager” and use that for your AoE. Also warriors can use Whirlwind. 

Also after you clear the instance up to the boss you could charge a couple people 1-2 gold to kill the boss for them in the already clear instance. 

Another thing is that while you're farming the instance you can bring a low level on another account or a friend and lvl his character up while you get money. 

Gathering Skills 
There are a number of gathering skills you can use to make money the first one I will go over is Skinning: 
Skinning is an easy to get up to 300 as you can use the skill while killing things to level up.  You can easily pick up large stacks of leather/hides to sell on the auction house as leather is used for a large number of recipes. 

Herbalism is a nice skill to get up as on each server there is a favored herb that you can sell on the AH for an amazing amount of gold.  On my server that is Groms Blood on other servers it will be different.  If you can find what your server favors then you can make a great deal of money. 

Mining is another nice gathering skill as you can get arcane crystals to transmute into arcanite bars and also get azerothian diamonds.  The arcanite can be used for your Arcanite Reaper as well. 

Special Farming Areas
Scarlet Spellbinders in Western Plaguelands:Drops 
These guys are lvl 56-58 humans that have a low chance to drop the Crusader enchant recipe which can sell for 200-400 gold.  Warning the mobs are easy to kill but watch out for other players. 
Blue Dragons in WinterspringDrops 
These dragons can drop the blue dragon sinew that hunters use for a special quiver and can sell for 200-600 gold depending on your server. 
Demons in southern WinterspringDrops 
These demons take a group to reliably farm but can drop the Eye of Shadow which can sell for 200-600 gold depending on your server. 
I will update this section later 
Outdoor Raid Bosses
Lord Kazzak Blasted LandsDrops 
Kazzak is a rather hard outdoor raid boss in the blasted lands who drops some very nice items.  Generally competition for the outdoor bosses is pretty high. 
Azuregos AzsharaDropsAzuregos is a large blue dragon boss with a nasty anti-zerg tool and for a experienced guild shouldn't be too hard.  Takes a bit of coordination but otherwise is fine. 
The four green dragonsAbout the dragons 
Lethon spawns throughout several zonesDropsLethon is one of four new outdoor bosses that are rather hard and generally for BWL guilds. 
Emeriss spawns throughout several zonesDropsEmeriss is yet another hard corrupted green dragon. 
Ysondre spawns throughout several zonesDropsDragon 
Taerar spawns throughout several zonesDropsDragon 
Special Instance Bosses
Blackrock Spire 
Goraluk AnvilcrackJed runewatcherBannok GrimaxeBurning FelguardCrystal FangGhok BashguudQuartermaster ZigrisSpirestone ButcherUrok Doomhowl 
Blackrock Depths 
Lord RoccorPanzor the InvinciblePyromancer LoregrainWarder Stilgiss 
 
 
 
 
 
Dire Maul 
MushgogLots of BoE world drop blues 
Skarr the UnbreakableLots of BoE world drop blues 
The RazzaLots of BoE world drop blues 
Stratholme 
Hearthsinger ForrestenStonespineThe UnforgivenTimmy the Cruel 
Scholomance 
None so far. 
Wanted Items and other things
 
Arcanite Transmute-Goes for 4-10 gold depending on the server almost always someone wants it.Takes a Arcane Crystal + Thorium Bar. 
Large Brilliant Shards-Used for a lot of high level enchants like +22 Intelligence and some others.Disenchanted from level 50+ Blues 
Fiery Weapon Enchant-This is a generally popular weapon enchant especially for rogues/warriors.Takes 4 small radiant shards and 1 essence of fire. 
Crusader Enchant-Generally sells for 30-120 gold popular with warriors for big slow 2handers.Takes 4 Large Brilliant Shards and 2 Righteous Orbs. 
Stronghold Gauntlets-Loved by warriors and many want this generally crafting cost is around 15-50.Takes a lot of materials. 
Lionheart Helmet-Another loved item for warriors about the same as Stronghold for crafting.Lots of Materials. 
Great Farming Spots
Ice Thistle Yeti'sWhere: WinterspringLevels: 56-59Skinning: YesHumanoidNotable Drops: Wicked Claws used for Lionheart Helmet 
MoonkinWhere: WinterspringLevels: 56-58Skinning: NoHumanoidNotable Drops: Wicked Claws used for Lionheart Helmet 
HippogryphsWhere: FeralasLevels: 43-47Skinning: UnknownBeast 
ScourgeWhere: Sorrow HillLevels: 52-54Skinning: NoUndeadNotable Drops: Scourge Stones 
GhostsWhere: WinterspringLevels: 53-57Skinning: NoUndead