Intro
No matter how you slice it, grinding is the fastest way to level in WoW. Maybe not the most fun, but it is much faster than questing.
Now, I'd say the best time to start grinding wouldn't be right off - You can get to 35 very easily questing, but after that, it's more of a pain in the as.
There are various good places to grind, so there is some overlapping.
Unfortunately, this applies mostly to damage dealers of the melee variety. Mages and Warlocks may be able to use them, but I don't have as much experience with them.
Warning: Alot of these areas are densely packed with mobs, so you are likely to get adds when you first begin clearing. However, the denser they are, that means the more spawns in the shortest distance - Exactly what you want when grinding.
Levels might be off by the order of 1 or 2, as your grinding ability might be different than that of the classes I have played. I apologize if they are, but I'm fairly sure these spots are mostly accurate.
Grinding areas will only support one person. Any more, and it will be alot slower for EXP. If someone is at the spot you planned on using, try somewhere else. I've tried to provide a good spread of places to try at different levels, but if you can't find one, you might just have to try later
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Preparing
If you're a melee class, I cannot stress the fact that first aid and food are godsends, and will reduce your downtime greatly. Love them. They are VERY useful.
You might also want to spend some money on +stat potions that last for an hour, as well as +hp regen. You're going to mostly be grinding on mobs that have good drops, so you'll have more to spend.
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Grinding
34-37
Venture Co. Meteorologists at Lake N. in Stranglethorn Vale
I love these guys. I mentioned them in the gold guide, and I really do love them. They have good drops, and they die fast. Hunters can sometimes even take two on at a time, as they like to stay at range and use their lightning bolt attack. Sic a pet on one, and take out the other yourself. They don't always stay at range, however, so your milage may vary.
36-38
The Naga on the islands in Desolace have low armor, and there are casters in the mix. Be wary of the wandering sea giants, though. One will walk across the island every now and then, and they'll kill you if they sees you.
37-39
The Murlocs by Grom'Gol in STV are excellent grinding. They have, as per usual, low defense, and take alot of damage. Some casters mixed in the fray as well.
36-46
In the Badlands, there are ALOT of rock elementals. They range from about 37 to 42 or so, if I remember correctly. They're sort of close to the Horde base, so Alliance might be weary of the area. They die fast, and they have some pretty good vendor drops. Higher level ones spawn by the Ogres camp, and have a chance of spwaning Rumbler, a named rock elemental. There's also a few quests right by the lower ones, so some extra EXP is fun.
40-43
The Scalding Whelps in the Badlands are fun to grind. They're basicly casters, low def and HP, high damage. They also have a chance of dropping a a Tiny Black Whelpling, which can go for 20-50 gold on some servers in the AH, and are pretty cool pets regardless.
40-46
The Wastewander humans in Tanaris are great grinding for a large level range. They start off at level 40 and 41, and ramp up to 44 or so, and then you can move to the pirates, which range from 44-45 (Or is it 46?). Chests spawns are all over the place too, so you can get some phat lewt. My gold guide has more in depth information on their location.
42-45
The Naga in Stranglethorn Vale are pretty easy to kill, though there aren't any caster mobs mixed in. I personally don't like this one, but it is viable.
44-46
There are some more Naga on an island in Feralas, south of Feathermoon Stronghold or whatever. I've never been there personally, but I've heard good things about it. A named naga spawns at the top of one of the caves, 24 hour spawn.
45-48
Also in Feralas, there are some Yeti in the Feral Scare Vale. The normal ones are about level 45, and they ramp up to level 48 or so. Pretty good grinding, but the range of movement is sorta large.
46-48
The ghosts in Azshara are great. I love them. I can't stress how awesome they are. They drop insane mageweave and silver, and are absolutely excellent to kill. The Litchlings are casters, and die even faster than the Apprentices, which go down FAST. I absolutely love these. However, on my server, several 57+ gold farmers also camp them fairly actively Sad
47-49
I've heard good things about the Dunemaul ogres in Tanaris, though I don't have much experience with the area.
47-49
I like elementals, and here are some more Smile. In the Searing Gorge Cauldron, and a little bit North of it, there are plenty to grind.
49-52
Deadwood Village in Felwood is decent grinding, and you can also use it to raise your Timbermaw faction. Not much I can say about it.
52-54
The Ogres in the Burning Steppes are pretty good. Same thing as with other ogres.
53-58
More ghosts! This time in Winterspring, on the frozen lake. I <3 these guys.
53-58
Running around in the Plaguelands killing undead. Raising Argent Dawn faction is a pain, so you might want to go ahead and do slightly slower grinding at the gain of being able to raise your faction at the same time. It's also possible to go to 60 doing this, but it's mind numbingly boring… But for some, the faction is worth it. You cannot gain faction from killing monsters when you are level 60, so if you plan on grinding them for the faction, you may really want to do this until 60
57-60
If you're a rogue, Blackrock Stronghold is the place to be. You will die a few times getting down a beat, but once you have the timing going well, you'll be dominating the area. Half the mobs there are casters, and the other half still have low armor compaired to most mobs of this level
While World of Warcraft's Public Test Realm has taken the backseat in favor of the Wrath of the Lich King beta, it doesn't mean Blizzard has totally ignored the test server altogether. The European servers have recently been wiped to make room for the new 3.0 build of the PTR, according to Blizzard poster Tigole.
- Updated description: Reduces the mana and focust cost of all shots and abilities by you and your pet by 20/40/60 while under the effect of Rapid Fire, and you gain mana equal to 50/100/150 of the damage you do under the effect of Rapid Killing over 6 sec. 
If you were lucky enough to register a 
