This is my first Guide, i hope you enjoy. Since English is not my primary language, bear with me.
This build works in Twisted Treeline and Summoners Rift
Maokai is both a great tank and ability user, this seems like a good thing to combine, when so many items let you have both.
Starting with low ability at start is not a problem, when your Sapling Toss hit level 3, you can toss them at Caster Minions and kill them right away.
While you wait for the saplings to get ready, you have lots of time to both harass opponent or other lanes.
When you get all your items you will see that he keep shining the more power you get, you can both tank and kill opponent easily.

As you can see, i win most of my matches with him, but they always surrender before we get to finish.

Greater Mark of Magic Penetration
Oh the penetration, it hits so hard, those runes with the Sorcerer's Shoes, such a good start and even a opponent tank will be scared.

Greater Seal of Cooldown Reduction
The fast cooldown makes it easy for you to use Sapling Toss very fast.

Greater Glyph of Ability Power
Ability pr. level, this will help you together with Rod of Ages the Ability is not important at start, but later on you got a lot, from all the items you have.

Greater Quintessence of Magic Penetration
Penetration, as i told before, this will make a tank shiver from the beginning of the match

Greater Quintessence of Knowledge
Mana is needed for your Sapling Toss, but still use it wisely

Greater Quintessence of Vitality
Remember you are also a tank, this will help you from start, to change the flow of the game before it even begins, low health will kill you too fast.

Clarity: You will need the mana, Doran's Ring will not be enough.

Flash: You do not start off with boots, solo or not you may be ganked. In the middle of a team fight, you may see that you cannot survive, flash away!

Sapling Toss as you can read is the most important thing you can ever have with Maokai this can take harass squishy opponents very easy.

--More to come--

Doran's Ring: This is a very good start, Sapling Toss, cost a lot of mana and on the other side also does more damage, with the extra health you start off with, you can ignore lots of damage (not tons tough) Sell this anytime you feel like it, because selling this to get a better item, may change your flow.

Rod of Ages: This item will guide you most of the time, meaning you will get lots of Ability Power, Health and Mana from this item. The faster you get this, the better.

Sorcerer's Shoes: The Spell penetration is very good and would do huge amount of damage toward you enemies, especially with your Vengeful Maelstrom when you see someone who tries to run from you.

Zhonya's Hourglass: As is told, if you are a half tank and half spellcaster, you can both go first into a team fight and end it with no problem. If any problems with health gets in the way of you tanking, you can activate its ability and invulnerable and untargetable and they end up focusing rest of your group and then you can activate your Vengeful Maelstrom right after to end it all.

Abyssal Mask: Reduce Magic Resist of nearby enemy champions by 20. Give you Magic Resist and Ability power to become more powerful and survive.

Rylai's Crystal Scepter: More health, you can not go wrong with that and your Sapling Toss slow your opponent.

Lich Bane/ Guardian Angel: If your team is already winning this match, Lich Bane is for more power and lowing damage taken from spells, but if you are against heavy damage, pop your Zhonya's Hourglass, get another life from Guardian Angel and finish the job.

I will change this page depending on what changes i find good or if any of you have a great idea, i may try it out and change the guide.
When i learn more about how to use this guide creator, guide will look better.