I used this build before the Maokai buff and was doing fairly well with it. Before the buff I was having trouble farming the creeps I needed for the items I wanted and wasn't doing enough damage to enemy champions for kills. I was completely reliant on other people for kills and assists.

This build transfered over with the buff to Maokai quite nicely as I am able to kill most squishy champions now with the damage he does with no ability power (Laning Phase).

This is my very first guide. (Edited as of June 2nd, 2011. Just reworded everything.)
Don't mock it until you try it. It works very well.

Pros:

[*] Amazing team support.
[*] Capable of withstanding lots of damage with Vengeful Maelstrom.
[*] With Sapling Toss you have amazing scouting capabilities.
[*] Capable of SOLO.
[*] Deals significant damage for a tanky character during laning phase.
[*] Shackle and Slow/Knock-up.
[*] Amazing at initiating.
[*] Can MAIN tank or OFF tank. VERY good MAIN tank.
[*] Best laugh in the game.

Cons:

[*] Very mana dependant and easily mana starved when using his greatest ability. Vengeful Maelstrom
[*] No escape.
[*] EXTREMELY slow.

While a tank you still want to do some decent damage and this allows it.

Greater Mark of Magic Penetration x9
Greater Quintessence of Magic Penetration x1

All this magic resistance sllows some leeway with tank items which allows you to get some damage and/or more health/armor/mana.

Greater Seal of Scaling Magic Resist x9
Greater Glyph of Scaling Magic Resist x9
Greater Quintessence of Scaling Magic Resist x2

All of the masteries are pretty straight forward. Maokai is first and foremost a tank/support then a damage dealer. This seems to be the only logical choice; while cool downs can be an issue staying alive to help your team is a better choice.

He is a good tank because of the three abilities he has.
Arcane Smash Allows him to stun/interupt and slow enemies to help the team escape or kill..
Twisted Advance Allows you to snare enemies to help the team escape or kill.
Vengeful Maelstrom Allows you to reduce damage done to MULTIPLE allies not just ONE. (For example Shen)

As Maokai I always start with a Regrowth Pendant and a Health Potion. This allows me to stay in the lane and attempt to out harass the enemy until I get 365 gold. At this point I always return to base buy Philosopher's Stone for maximum potential gold and for mana because if you are harassing you are low on mana. I use Teleport to quickly return to the lane and miss very little experience.

With those 2 items I continue to harass and get creep kills. At level 4-5 you should be doing some nasty damage to kill a champion if you hadn't already. There is no specific order for the items I chose but the core is a must. Make a judgement call on when you get boots but I normally go straight for the Philosopher's Stone and 2 Heart of Golds. THEN the Mercury Treads.

Depending on your playstyle you may get the boots earlier and go ganking. If I manage to obtain first blood I get Boots after Philosopher's Stone because he is hella slow.

END GAME ITEMS ARE ALWAYS SITUATIONAL!
Okay so, after the initial core items, Banshee Veil and Frozen Heart its up to you.
Some options are:
Archangel's Staff for some ability power if you've got another tank or if you are so far ahead of the damage curve of the other team it doesn't matter.
Guardian Angel Self explanitory really. Coming back from the dead is always a good thing.
Warmog's Armor Need more health?
Plus many others but these are some of the things I go with after Frozen Heart. Each one item leads to a different route. IE more damage or more tank.

Explanation for starting items:
With the 2 Heart of Golds and Mercury Treads you will have enough magic resistance and health to take the damage of the enemy team. Not only that with the Gold per 5 items like these you are able to go around constantly ganking without having to worry about farming creeps. Staying ahead of the damage curve of the other team is another reason this is a good way to start off.

I am prepared to be flamed over my choice of skill order.

Personally I feel as if Maokai's Vengeful Maelstrom takes way too much mana and he doesn't have the mana to effectively use it at level 6.

That is why I rotate between gettin Sapling Toss and Twisted Advance.
Sapling Toss for the Damage is does to enemies, at skill level 4 it can 1 shot a creep wave with no ability power.
Twisted Advance for the ability to keep the enemy champion in place to take full damage from Sapling Toss.

At level 11 I get 1 rank of Vengeful Maelstrom and then I max Arcane Smash for the ability slow and STOP channeled spells because it is a knock-up/stun. Which is always situational. You may need it sooner to stop a Nunu & Willump/ Katarina/ Fiddlesticks for example.

Call me conceited but I personally think there is no better options than what I have suggested. Especially for the build I have. Teleport is MANDITORY because he is so damn slow.

However while I think Exhaust is amazing for Maokai, because it is another way to help your team or snare for a kill, there is other options.

Clarity - He DOES for through a lot of mana. This would enable him to have his Vengeful Maelstrom at level 6.
Ignite - He does some insane damage while laning and this could be a way to finish someone.
Flash or Ghost - Any escape is always an okay option.
Smite - I thought I would mention this because I've seen a Maokai jungle once before. Didn't pay too much attention to how he did it but he was successful in staying alive and progressing properly. I would suggest against this.

Everything else really isn't worth mentioning.