Engines...

When the game was young, deck usually consisted of just the best cards, essentially leaving deck building an unused factor.
Ng Yu Leung played 27 single copies of his World Championship 2003 winning deck.
2007 this number was 19. The only kind of combo was Monarch + Treeborn and Card Trooper + Machine Duplication.
2011 this number was 9. The deck was an archetype, consisting of 6 set up cards to play 5 boss monsters.

As the game developed, 'engines' were developed. An engine is essentially a set of cards which work together, meaning less broken splashable single cards needed printing.
Engines had their prime during the TeleDAD era, were the Destiny Draw-Emergency Teleport-Dark Armed Dragon had ridiculously high synergy, and raw power.

The goal of this post is to bring forth some of the engines that still exists, how to play them, and what they accomplish.



Rabbit

2-3 Rescue Rabbit
3 Level 4 or lower Normal Monster
2-3 Level 4 or lower Normal Monster
0-3 White Elephant's Gift
0-1 Pot of Avarice
0-3 Common Charity

Total space is 7 to 16 cards, but in reality around 9-11 cards.

The engine gives speed and to some extent control to decks that lacked it. The engine is big, meaning it is not easily splashed but can still be.
The choice of Normal Monsters are purely up to you, but a common option includes playing Gene-Warped Warwolf and Mad Dog of Darkness, along with Reborn Tengu and Horn of the Phantom Beast. A deck which can Special Summon tuner monsters can also make use of Rescue Rabbit.
Good examples of these kind of tuners includes Plaguespreader Zombie, Unknown Synchron, Glow-Up Bulb, and so forth.
Another somewhat unexplored prospect is playing the Elemental Heroes along with Rescue Rabbit. White Elephant's Gift helps drawing the cards you need, and even if they are destroyed during the End Phase, you still have Miracle Fusion readily available.
Of course, the most used way of abusing Rescue Rabbit (or the most abusing way of using...) is to summon Level 4 Dinosaurs and overlay Evolkaiser Raiga. Having a 'free' Solemn Judgment is always nice, and along with Glow-Up Bulb you have easy access to Naturia Beast and Trishula, arguably the two of the best currently available Synchro monsters.
Playing level 3 monsters are also good, as they gives you access to Leviair the Sea Dragon for two potential extra Rescue Rabbit uses.
As a last word on this topic, it is also possible to use Rescue Rabbit for powerful Synchro summons.
There are Level 1 and 3 Normal Tuners, Flamvell Guard, Water Spirit, Tune Warrior and Genex Controller. Great combos with Dandylion, Gravekeeper's Spy, Cyber Dragon, Agent of Creation - Venus, Reborn Tengu, even Giant Germ can be fatal.
Also, Flamvell Guard can be used for Cards of Consonance, Genex Controller can be used with Allure of Darkness, Genex Undine. Tune Warrior has a mighty 1600 ATK (you laughed there didn't you?) and could potentially be searched by Reinforcement of the Army, Water Spirit can be used for Spiritual Water Art - Aoi and Moray of Greed.

The thing is that most Normal Monster actually have some way to be used from the hand, making them less useless then what might seems to be the case.

I can see almost any deck having a useful combo with this card. Karakuri, for example, can play Cyber Dragon, then bring out two level 3 Tuners and make Bureido, Special Summon a Level 4 non-Tuner Karakuri and use the second tuner for Burei.
I can't possibly write down everything, but now you know: Rescue Rabbit is powerful.



Dandy/Debris

1 Glow Up-Bulb
0-1 Lonefire Blossom
0-1 Spore
0-1 Dandylion
0-1 Debris Dragon
0-1 Naturia Cosmobeet
0-1 Tytannial, Princess of Camellias
0-1 Mystic Tomato
0-1 Pot of Avarice
0-1 One for One

This one is interesting. In my opinion, it is possible to splash 1 Lonefire, 1 Spore, 1 Dandylion, Debris Dragon, 2 Card Trooper and One for One into almost every deck. These small critters give advantage, as far as I am concerned, Debris Dragon is a really broken card.
I mean, Debris Dragon can now with the release of Orient Dragon prove lethal. That Ryko that used to give you Iron Chain can now eliminate any Synchro your opponent controls.
Furthermore, you can even use it in X-Saber to bring back Darksoul and nuke.
If you have a Doppelwarrior in your hand, you can freely Normal Summon Debris Dragon, bring back a Level 1 Tuner, Special Summon Doppelwarrior and make Orient Dragon. Use a Token and the tuner for Formual Synchron, then you can make Trishula. You used two cards to gain a Level 9 Synchro, removing a card, a card in your opponent's hand, a Synchro monster your opponent controlled, a card in their Graveyard, and drawing a card. Naturia Comsobeet is underestimated. Your opponent will often find themselves staring in the face of a wicked Synchro like Naturia Beast or Naturia Barkion, or a more generic one like Brionac or T.G. Hyper Librarian.
This kind of engine gives you easy access to almost all different levels of Synchro Monsters, while also being relatively solid on their own. Debris Dragon also has 2000 def, which should not be underestimated. I have actually ran into decks which haven't had any counter to it besides Torrential Tribute.



Junk/Doppel

2-3 Junk Synchron
2-3 Doppelwarrior
0-1 Quickdraw Synchron
0-1 Synchron Explorer
0-1 Level Eater
1 Reinforcement of the Army
0-1 Foolish Burial
0-3 Tuning

This is similar to the above engine, but none of the key cards are limited. Junk Synchron is a really good card, being able to bring out a Level 4-5 Synchro by just using your Normal Summon. Unlike Debris Dragon however, his useful targets used to be more limited (only Tuningware).
But with the release of Doppelwarrior, I strongly believe they are the better pair. You can't do a Black Rose, but you can do a Junk Warrior, Junk Destroyer, Junk Archer, Frozen Fitzgerald, and so on. Junk Synchron also has heavy support in being a DARK Warrior, unlike Debris Dragon's WIND Dragon, basically only qualifying it for Cards of Consonance and Dark Simorgh. Junk Synchron can be used with or for Allure of Darkness, Tuning, Reinforcement of the Army, Synchron Explorer, The Warrior Returning Alive, Chaos Sorcerer, Black Luster Soldier or Dark Armed Dragon.
The main reason Debris Dragon saw more play was because Dandylion is a good card, and with the Plant engine with double Lonefire you had better opening plays. Nearly all of the best Sychros are Dragons anyway, Scrap Dragon, Stardust Dragon, Trishula, Gungnir and so on.
I expect some kind of deck using Junk Synchron to be top tier. It is a really good card.



Blackwing

1 Blackwing - Zephyros the Elite
0-1 Blackwing - Gale the Whirlwind
0-3 Blackwing - Blizzard the Far North
0-3 Blackwing - Vayu the Emblem of Honour
0-3 Armageddon Knight
0-3 Icarus Attack

This one is really interesting. I mean, you could technically play 3 Armageddon Knight, 1 Zephyros and 3 Blizzard, along with personal preference of Icarus Attack. By simply playing Armageddon t1, you have access to Icarus Attack as well as Blizzard for the rest of the game. Spamming level 6 Synchros might not be such a broken choice, but Armed Wing, Brionac and Orient Dragon are really good cards. Blizzard can also be recycled using Recurring Nightmare. Reccuring Nightmare may also be used on T.G. Warwolf, meaning there is a chance to splash them in as well.
If your level 6 survives, you can make a level 8 instead.
Playing Doppelwarrior can also be beneficial, as you can make a Level 8 instantly. If both the tokens survives, another level 8 is ready.
The main problem with these cards are the lack of flexibility offered. You are basically very limited in Synchro summoning capabilities.



Agents

3 Mystical Shine Ball
2-3 The Agent of Creation - Venus
2-3 The Agent of Mystery - Earth
0-3 Master Hyperion
0-2 Archlord Kristya
0-3 Cards from the Sky

Taking out the main core from what many declare the best deck in format. The optimized core seems to be 3 Earth, 2 Venus, 3 Shine Ball. Might want to play White Elephant's Gift as well.
This core is really good, and doesn't even need Hyperion to perform. You can still do t1 Gachi Gachi Gantetsu or Daigusto Phoenix, or t2 Trishula if you drop Earth t1.The fact that Earth is a Tuner makes it really good. All of these are also LIGHT, meaning you only need DARKs to complete the Chaos Circle. As a side note, there are a lot of splashable Dark monsters, such as Sangan, Gorz, Unknown Synchron, Tragoedia and whatever, meaning the Light side are more of a problem.
The biggest problem with this deck as I see it is of course the Shine Balls. They can, unlike the mostly powerful monsters used in Rescue Rabbit, easily be drawn as completely dead cards, and are in those cases a bit harder to get rid of.
However, if you can successfully play down Earth and then Venus, you can make astounding things.
Trishula + Shine Ball
Gachi Gachi Gantetsu x 2 + Venus
Gachi Gachi Gantetsu + Ancient Sacred Wyvern
Gachi Gachi Gantetsu + Armory Arm + Venus
Armory Arm + Daigusto Phoenix + Venus
Level 8 + Venus

The possibilities are intriguing. Together with a second tuner you have really broken opportunities.



HERO

1 Elemental Hero Stratos
1-3 Elemental Hero Neos Alius
0-1 Elemental Hero Voltic
0-1 Elemental Hero Ice Edge
0-2 Destiny Hero - Malicous
0-2 Level 4 or lower Destiny Hero of you choice
0-2 Destiny Draw
1 Reinforcement of the Army
0-1 Future Fusion
2-3 Miracle Fusion
0-3 A Hero Lives
0-2 Super Polymerization

This one got big when Absolute Zero was released. Destiny Draw was at 3, so playing it along with Destiny Hero - Malicious, Future Fusion, Treeborn Frog and Deep Sea Diva was anticipated. The decks did generally not perform very well, but that is another story.
You can chose to skip out on the E Heroes, or skip out on the D Heroes. Generally going in one direction is more preferable than not.

The biggest problem is consistency. The D Heroes and E Heroes, with the exception of Stratos, generally perform really bad on their own. You must also dedicate a bit of Extra Deck space to Fusions if playing the E Heroes. When playing the E Heroes you add a bit of inconsistency, but gain a lot of hitting power, especially to deck that used to lack it.



Frog

2-3 Treeborn Frog
0-3 Swap Frog
0-1 Ronintoadin
0-3 Genex Undine
0-3 Genex Controller

This engine could be classified as dead in decks except Frog Monarch. The banning of Fishborg Blaster was a grave hit to the usability of this engine, but it is still solid and provides a simple wall or endless tribute fodder.




That's it for this time. I know I must have missed a lot of engines, please remind me!