Hearthpwn

EDIT (12/11/14): Okay guys, I get it -- this deck is terrible. I made it before GvG was released, and lots of people including me thought it would work, but it didn't take much testing to find out it doesn't. I have no idea why it's remained popular despite it's ineffectiveness, but it has. If anyone has an amazingly effective Zoo deck, let me know via PM or here -- I will make it popular by replacing all the cards listed for this deck with the ones from yours, so a good deck can be played by thousands of people and not this crap-tier one. Please provide statistics on your deck, preferably from a Hearthstone tracker.

Cards that synergize with Mechs: 17
Cards that synergize with Undertaker: 8
Cards that synergize with Hobgoblin: 10

The thesis of this deck is Triple Synergy. Most of the cards synergize in multiple ways. Playing stand-alone value cards just doesn't cut it in Hearthstone anymore. Great decks these days are full of cards that combo and interact with each other to gain value out of thin air. This deck presents 3 mechanisms and makes each card played likely to get synergy value.

This deck is incredibly aggressive, with the stampede starting quite early.
Early Game: Develop Undertaker & Mech Synergy
Mid Game: Develop Hobgoblin Synergy.
Late Game: Buff/Finishers.

Mulligan for Undertaker and cheap Deathrattles. Over 50% of the time, on turn 4 you will have drawn Hobgoblin (1/15 chance, 8 cards drawn, 8/15 > 0.5 > 50%), so unless you have coin I wouldn't keep Hobgoblin as a mulligan pick. Keep Hobgoblin if you also have Undertaker and a Deathrattle for him.

Card-by-Card breakdown:

2x Clockwork Gnome - Undertaker fuel & Mech synergy. Spare Parts are extremely useful due to Zoo being so good at board control. You will always find utility in whichever part you get.

2x Cogmaster - Hobgoblin fuel & Mech synergy. If you play this with Hobgoblin and a Mech on the board, Hobgoblin buffs this first and then +2 Attack gets applied from the back on top, making this a 5/4 for 1 mana.

2x Leper Gnome - Undertaker fuel. Solid aggro-focused 1 drop.

2x Annoy-o-Tron - Mech synergy & Hobgoblin fuel. This harasses and kills any early game drop with 2 health. In a lot of ways it's a mini Sludge Belcher, so even lategame it's a good card if you need a turn to stabilize.

2x Undertaker - A central theme of the deck, Undertaker has 8 targets to get buffed by, and almost all of those targets are 2 mana or less, making early game ALL about Undertaker.

2x Darkbomb - Replaces Soulfire. 1 less damage, 1 more mana, no discard. Seems fair.

2x Haunted Creeper - UNDERTAKER AND HOBGOBLIN FUEL. Easily one of the best cards in this deck as a result. Really sticky minion, battle tested to be great in Zoo, and now it's even better.

1x Ironbeak Owl - I have played a LOT of Zoo, and have found the utility of a silence to be very important. Also, if you're getting TOO much value out of Hobgoblin, you might want to have mercy on your opponent and silence him ;)

2x Mechwarper - Mech synergy. No one doubts the power of this Mech. Probably in the top 5 of strongest GvG cards.

2x Micro Machine - Hobgoblin fuel & Mech synergy. Plays as a 3/4 with Hob, and if it survives until your turn again, comes back to you as a 5/4.

2x Harvest Golem - Undertaker fuel & Mech synergy.

2x Tinkertown Technician - Mech synergy. If you don't have a mech by turn 3, I'd be shocked. This card gets so much value on average that it had to be included.

2x Defender of Argus - This will vitally allow you to protect your Hobgoblins and make the board more difficult for opponents to trade into.

1x Enhance-o Mechano - Mech synergy. You'll often get this out turn 3 or turn 4 and buff two other minions. As long as you buff 2 minions, this card gets great value. Especially with one getting Taunt and another getting Windfury.

2x Doomguard - Because how could you not? It may be a mistake since this deck relies so much on synergies, you may not want to play this for fear of losing your Hobgoblin that you want to get combos going for. So we'll see. If I were to replace Doomguard, I'd choose Mimiron's Head and Loatheb.