So many choices!

So let's say you're a pretty little kitty--I mean fierce--let's say you're a fierce little kitty running around Azeroth. While you're running around you see a boss mob and you think to yourself, "Self, I know when I make him bleed his armor doesn't do anything to help him. But wouldn't it be awesome if my claws and teeth could go right through his armor like Ragnaros's mace through butter? Mmmm...butter...
"Anyway, I wonder what that would take?"

Interesting question, and one I've been wondering for myself. So I did some research. I'd like to share it with you. Because I'm nice like that. And I'm high on Girl Scout cookies. But I don't share those. So don't ask.

I'm not going to go over the full math and mechanics of Armor Penetration (ArPen) with you because there's eleventy-thousand write-ups out there for it already and I doubt I could do a better job explaining it than most of them. What I will give you is the following numbers:

Hard cap for ArPen is 1,399.57272. But the game engine rounds up to whole numbers, which means you need 1,400 ArPen for your teeth and claws to completely ignore a raid boss's armor.

"But wait," you say. "I'm a Druid and I have this nifty Faerie Fire spell that reduces an enemy's armor." How astute of you. Gold star. Because you're so awesome, your ArPen cap is a mere 1,330. (Edit: Silly mistake on my part, getting Armor Reduction confused with Armor Penetration. I should know better. You're still awesome, but Faerie Fire is no help here. Cap is still 1,400. Soft-cap numbers below have been modified appropriately.) On top of that you'll want to make sure you maintain your 263 Hit (230 if you have a Draenei in your raid) and 132 Expertise rating caps so that you're not missing or having your attacks dodged.

So how to get there? Can you get there?

Actually, yes. Yes you can. I found one combination of 15 ArPen-granting, non-Heroic Mode items currently in the game and available for your Druidy self to wear that, when gemmed right, will top off all 3 mentioned stats and leave you with 2 gems slots to spare. But you have to be very dedicated, really lucky with the /roll, and in a guild capable of running ICC 25 to accomplish that.

Good news: You don't have to hit the hard cap! There's a number of soft-caps out there. How do you determine the various ArPen soft-caps? Well, you first have to obtain and equip one of the following:
Grim Toll (788 Soft-cap)
Mjolnir Runestone (735 Soft-cap)
Needle-Encrusted Scorpion (722 Soft-Cap)
Deathbringer's Will (600 ArPen proc + 155 ArPen on Trinket => 575 Soft-cap)
(Edit: Bornakk posted on Jan 6 that the ArPen proc on Deathbringer's Will had been replaced with a Haste proc. I missed that.)

Then you take the potential ArPen proc and subtract that from 1,400. Because I'm still nice (and high) I've already done it for you.

Most of those are pretty obtainable without a crapton of effort.

I went through and built up a list of Druid-equippable gear that you can get from the following sources:

• ICC 25 Normal
  
• ICC 10 Normal
  
• ToC 25 Normal
  
• ToC 10 Normal
  
• Emblems of Frost
  
• Emblems of Triumph
  
• ICC Heroic Dungeons
  
• Ulduar/Naxx bosses you might be killing during weekly raid quests
  
• Leatherworking patterns

I feel the majority of players will have access to these sources. Some are definitely harder to obtain than others, but it's stuff that's getting run by guilds and pugs on a regular basis.

So get ready. Here's your list:

ITEM	ARPEN	HIT	EXP	GEMS	DROPS FROM
Chest
Sanctified Lasherwave Raiment	92	0	0	R,Y	Vanquisher's Mark of Sanctification
Choking Hauberk	89	59	0	R	Marwyn (H HoR)
Lasherweave Raiment	80	0	0	R,Y	95 Emblems of Frost
Chestguard of the Frigid Noose	79	0	0	R,Y,B	Lady Deathwhisper (ICC 10)
Knightbane Carapace	74	82	0	R,Y,B	Leatherworking
Armor of Shifting Shadows	59	0	0	R,Y	Twin Val'kyr (ToC 10)
Boots
Taldarams' Soft Slippers	71	0	0	2R	Blood Princes (ICC 10)
Frostbitten Fur Boots	64	0	0	2Y	Marrowgar (ICC 25)
Blighted Leather Footpads	63	0	0	X	Escape Event (H HoR)
Treads of the Icewalker	59	67	0	Y	Faction Champs (ToC 25)
Gloves
Aldiana's Gloves of Secrecy	80	0	0	R,Y	Rotface (ICC 25)
Sanctified Lasherweave Handgrips	80	0	0	R	Vanquisher's Mark of Sanctification
Lasherweave Handgrips	71	0	0	R	60 Emblems of Frost
Cat Burglar's Grips	64	0	0	2Y	60 Emblems of Frost
Scourge Stranglers	63	0	0	R,Y	Deathbringer Saurfang (ICC 10)
Gloves of the Silver Assassin	54	0	0	R	Faction Champs (ToC 10)
Helm
Sanctified Lasherweave Headguard	92	0	0	R	Vanquisher's Mark of Sanctification
Discarded Bag of Entrails	88	0	0	Y	Professor Putricide (ICC 10)
Lasherweave Headguard	80	0	0	R	95 Emblems of Frost
Hood of Lethal Intent	74	82	0	Y	75 Emblems of Triumph
Pants
Sanctified Lasherweave Legguards	108	0	92	R,Y	Vanquisher's Mark of Sanctification
Bladeborn Leggings	100	0	0	R,Y,B	Leatherworking
Lasherweave Legguards	96	0	80	R,Y	95 Emblems of Frost
Ivory-Inlaid Leggings	60	0	0	2R, Y	Blood Queen Lana'thel (ICC 10)
Shoulders
Sanctified Lasherweave Shoulderpads	72	0	0	R	Vanquisher's Mark of Sanctification
Skinned Whelp Shoulders	71	0	0	2R	Valithria Dreamwalker (ICC 10)
Lasherweave Shoulderpads	63	0	0	R	60 Emblems of Frost
Duskstalker Shoulderpads	59	0	0	Y	45 Emblems of Triumph
Malfurion's Shoulderpads of Triumph	59	0	0	Y	45 Emblems of Frost, 1 Trophy of the Crusade
Malfurion's Shoulderpads of Conquest	52	0	0	Y	30 Emblems of Triumph
Belt
Vengeful Noose	80	0	0	R,Y	60 Emblems of Frost
Astrylian's Sutured Cinch	64	0	0	R,B	Professor Putricide (ICC 25)
Bracers
Toskk's Maximized Wristguards	60	0	0	Y	Deathbringer Saurfang (ICC 25)
Bracers of Swift Death	42	0	0	B	Leatherworking
Chewed Leather Wristguards	41	46	0	X	Ick (H PoS)
Cloak
Recovered Scarlet Onslaught Cape	52	0	0	B	50 Emblems of Frost
Drape of the Drakerider	36	0	0	X	Razorscale (Uld 25)
Amulet
Sindragosa's Cruel Claw	60	0	0	B	Sindragosa (ICC 25)
Rimetooth Pendant	53	0	0	Y	Sindragosa (ICC 10)
Collar of Ceaseless Torment	42	0	0	X	Beasts of Northrend (ToC 10)
Broach of the Wailing Night	46	0	0	X	19 Emblems of Conquest
Rings
Frostbrood Sapphire Ring	60	0	0	R	Valithria Dreamwalker (ICC 25)
Saurfang's Cold-Forged Band	53	0	0	Y	Deathbringer Saurfang (ICC 10)
Dexterous Brightstone Ring	50	0	0	X	35 Emblems of Triumph
Cindershard Ring	46	42	0	X	Ignis (Uld 25)
Planestalker Signet	39	38	0	Y	Jaraxxus (ToC 10)
Band of Stained Souls	36	0	0	X	Tyrannus (H PoS)
Weapon
Tainted Twig of Nordrassil	90	0	0	R,Y	Lich King (ICC 10)
Lupine Longstaff	82	0	0	B	Twin Val'kyr (ToC 25)
Trinkets
Deathbringer's Will	155	0	0	X	Deathbringer Saurfang (ICC 25)
Needle-Encrusted Scorpion	0	83	0	X	Devourer of Souls (H FoS)
Mjolnir Runestone	0	0	0	X	Thorim (Uld 10)
Grim Toll	0	83	0	X	Gothik, Grobbulus, Heigan, Maexxna (Naxx 25)
Banner of Victory	84	0	0	X	Paletress (N ToC5)

I list the Alliance drops from ToC, but the Horde equivalents drop off the same bosses and have the same stats. Also note that the Vanquisher's Mark of Sanctifications drop from Deathbringer Saurfang, Professor Putricide, Blood Queen Lana'Thel, Sindragosa, and the Lich King in ICC 25. To get the items that require them, you also need to have purchased and trade in the lower-level equivalents.

So now there's a whole slew of possibilities. You can look at the above list of gear, set your goals, determine what you have a reasonable shot of getting, and go from there. But let's look at a couple basic scenarios.

First, some assumptions:

1. I'm going to use the Needle-Encrusted Scorpion (722 Soft-cap) in all the examples. It's probably the easiest of the four trinkets to get and the one I assume most people will have.
   
2. These numbers don't include nomming some Hearty Rhino. It's a good source of ArPen, but one that needs constant replenishing. So I don't want to automatically add it to the calculations.
   
3. Sunder Armor and/or Expose Armor are not considered. This is for your personal ArPen cap. Obviously if you expect to always have these debuffs up in your raids, you can lower your cap numbers by another 280. (Edit: Same silly mistake as with FF.)
   
4. I'm not going to worry about hitting the Hit or Expertise caps. I'll get you to the ArPen cap in these lists. You're responsible for figuring out how to use other gear, gems, and enchants to hit the other caps.

Okay, so first example. Let's say you want to be capped before ever stepping foot into a raid. Just farming badges and Heroic drops. You can do that!

Here's one way:
Head - Hood of Lethal Intent (75 Emblems of Triumph)
Shoulders - Duskstalker Shoulderpads (45 Emblems of Triumph)
Chest - Knightbane Carapace (Leatherworking)
Bracers - Bracers of Swift Death (Leatherworking)
Belt - Vengeful Noose (60 Emblems of Frost)
Boots - Blighted Leather Footpads (Escape Event (H HoR))
Finger - Dexterous Brightstone Ring (35 Emblems of Triumph)
Finger - Band of Stained Souls (Tyrannus (H PoS))
Trinket - Banner of Victory (Paletress (N ToC5))
8 Fractured Cardinal Rubies
Pretty reasonable and only one piece of Frost Emblem gear. Though if you're not raiding that 60 will take a month of running a daily random.

Here's another:
Head - Hood of Lethal Intent (75 Emblems of Triumph)
Shoulders - Duskstalker Shoulderpads (45 Emblems of Triumph)
Chest - Knightbane Carapace (Leatherworking)
Bracers - Bracers of Swift Death (Leatherworking)
Legs - Lasherweave Legguards (95 Emblems of Frost)
Boots - Blighted Leather Footpads (Escape Event (H HoR))
Finger - Dexterous Brightstone Ring (35 Emblems of Triumph)
Trinket - Banner of Victory (Paletress (N ToC5))
9 Fractured Cardinal Rubies
This one takes fewer pieces of gear, but the trade-off is going from a
60 Frost Emblem piece to a 95 Frost Emblem piece, which will increase
the amount of farming needed.

You can go this way, too:
Head - Hood of Lethal Intent (75 Emblems of Triumph)
Shoulders - Duskstalker Shoulderpads (45 Emblems of Triumph)
Chest - Knightbane Carapace (Leatherworking)
Bracers - Bracers of Swift Death (Leatherworking)
Legs - Bladeborn Leggings (Leatherworking)
Boots - Blighted Leather Footpads (Escape Event (H HoR))
Finger - Dexterous Brightstone Ring (35 Emblems of Triumph)
Finger - Band of Stained Souls (Tyrannus (H PoS))
Trinket - Banner of Victory (Paletress (N ToC5))
7 Fractured Cardinal Rubies
No Frost Badge gear in this one, but it does have the Bladeborn Leggings, which are going to run you a lot of Frost Emblems or a small fortune on the AH for the Primordial Saronite.

Maybe your guild is running 10-man raids, so you have access to those pieces. Let's say, for instance, that ToC 10 and Lower ICC 10 are pretty easy for your group to clear. Plus the Frost Emblems will be coming in a little faster, so maybe you can depend on having more than one piece of Emblem gear available to you. This is one combination you can try for:
Shoulders - Lasherweave Shoulderpads (60 Emblems of Frost)
Chest - Knightbane Carapace (Leatherworking)
Bracers - Chewed Leather Wristguards (Ick (H PoS))
Legs - Bladeborn Leggings (Leatherworking)
Boots - Blighted Leather Footpads (Escape Event (H HoR))
Neck - Collar of Ceaseless Torment (Beasts of Northrend (ToC 10))
Finger - Planestalker Signet (Jaraxxus (ToC 10))
Finger - Band of Stained Souls (Tyrannus (H PoS))
Trinket - Banner of Victory (Paletress (N ToC5))
9 Fractured Cardinal Rubies

Here's a good one that gives your your 2-piece T10 Bonus:
Head - Hood of Lethal Intent (75 Emblems of Triumph)
Shoulders - Duskstalker Shoulderpads (45 Emblems of Triumph)
Chest - Chestguard of the Frigid Noose (Lady Deathwhisper (ICC 10))
Bracers - Chewed Leather Wristguards (Ick (H PoS))
Belt - Vengeful Noose (60 Emblems of Frost)
Legs - Lasherweave Legguards (95 Emblems of Frost)
Finger - Saurfang's Cold-Forged Band (Deathbringer Saurfang (ICC 10))
Finger - Band of Stained Souls (Tyrannus (H PoS))
Trinket - Banner of Victory (Paletress (N ToC5))
6 Fractured Cardinal Rubies

And if you're in a guild that has full access to 10- and 25-man raid content? Here's one that will work and get you your 4-piece T10 bonus:
Head - Sanctified Lasherweave Headguard (Vanquisher's Mark of Sanctification)
Chest - Sanctified Lasherwave Raiment (Vanquisher's Mark of
Sanctification)
Bracers - Toskk's Maximized Wristguards (Deathbringer Saurfang (ICC 25))
Hands - Sanctified Lasherweave Handgrips (Vanquisher's Mark of
Sanctification)
Belt - Astrylian's Sutured Cinch (Professor Putricide (ICC 25))
Legs - Sanctified Lasherweave Legguards (Vanquisher's Mark of
Sanctification)
Boots - Taldarams' Soft Slippers (Blood Princes (ICC 10))
Trinket - Deathbringer's Will (Deathbringer Saurfang (ICC 25))
0 Fractured Cardinal Rubies

All-in-all, there's over 15 million combinations of the gear I listed above, and literally tens-of-thousands of them that will get you to exactly any of the given soft-caps. I wrote a small program to find these combinations based on certain parameters, but you can probably whip out a pad and pencil and get pretty close on your own.

Just remember that if you have to go over the soft-cap, you don't want to go over by too much. Every point you go over is a point wasted off your proc and a point that could have been spent on other itemization.

Also, it's worth noting that you can get the Sweet Perfume Broach from Apothecary Hummel during Love is in the Air right now. It's an exact duplicate of the Broach of the Wailing Night. So if you want that but don't want to drop the 19 Emblems of Conquest on it, start hitting him up!

Now if you'll excuse me, I have cookies calling my name.

Happy hunting!