This is a skeleton guide for a really, really annoying Maokai. His job is to wade through all the damage that the enemy can throw at him, and also clear gank spots. Not very good damage output, but massive survivability and very good harrassment. Also, since all of the runes used for this build are flat bonuses, it is vital to initiate fights early game in order to secure the advantage for mid and late game.

Since this is a straight-out sandbag build, I've put in only armor, magic resist, and health runes. The reason why there isn't any dodge or health regen is because, quite frankly, it won't make much of a difference in a fight. You can modify the amount of magic resist or armor depending on whether you want to go physical or magical tank, but I choose a balance between the two, since stacking a single stat won't be effective against balanced team comps.

This is probably the freest area of this guide (meaning that you have the most room for modifications). There's some magic resist and armor, plus health and mana regen, and a bit of experience and gold, and that sweet 4% damage reduction from Tenacity.

Since you'll be spamming Saplings as much as possible, start off with a Sapphire Crystal and a Health/Mana pot to keep you in lane. Return immediately as soon as you have enough for your Catalyst, and come back again for your Rod of Ages. Return once more for Mercury's Treads. At this point, you've got three options. If the enemy team is building AD, then get a Warmog's. If it's AP, then Force of Nature. If it's a balance between the two, Banshee's. Return for the other two, and wrap it up with a Frozen Heart.

You're a bushwhacker. That means that you want your Sapling Toss and Twisted Advance maxed ASAP. Again, you've got a little leeway here, but whatever you do, max Sapling Toss first. Your other skill sequence depends on your team's game plan. For ganks, Twisted Advance. For laning, Arcane Smash. Obviously, you want to max Vengeful Maelstrom as soon as it becomes available.

You're a tank. Tank being the key word. Therefore, you either want something that'll boost your defensive capabilities or assist the rest of the team. I normally go with Clairvoyance and Clarity, but it's really your choice. I'd recommend Heal and Clarity, which are very powerful early-game spells, but Heal become more or less obsolete near end-game. You can also try out Rally/Fortify, or any combination of the above listed. Whatever suits your tastes.

EDIT: I now go Fortify/Clarity, since it keeps you in lane for extended periods of time, which enables you to get ahead in exp/gold whenever your laning partner blue pills, and can also turn a tower-diving gank into a punishing multi-kill (if you have the mastery), and also keeps your towers safe from getting rushed if (God forbid) you recall during an enemy push.

All right, the basic gameplan is to harass. Learn how to use your Saplings effectively and you will dominate your lane. (Just FYI, this entire section is written assuming there is no jungler on your team.)

Lanes:
If you're going top/bot, ward both the brush off to the side of the lane and the brush in the river. If it's mid (you/your team is being ******ed if this is true, or you're trolling) then ward the side paths on your side (The small pathways that are off to the side right before the river) and at least 1 side of the brush in the river.
MAP AWARENESS!!! ALWAYS use your mini-map, and instantly call mias. This is one of the things that pisses me off most about tanks. It's YOUR job to make sure no one else gets dies. If you feel uncertain about an area/buff that you want to get, USE CLAIRVOYANCE! Buy wards and plant them near high-traffic spots, such as the river.

Dragon Control:
I would recommend that you take bot if you can, since you can always keep an eye on Dragon. Dragon control is very, very important throughout the game, as it provides 190 gold and 400 xp to the entire team of the player that killed it, despite being relatively easy to kill.

Buff Control:
Make sure you keep watch over your side's buffs at all times. ESPECIALLY when there is a jungler on the enemy team. Again, map awareness! Map awareness leads to informed decisions. Informed decisions beget advantageous fights. Advantageous fights (usually, I've seen 1v5 ganks that turn into a penta-kill for the target) lead to gold/xp advantage. Gold/xp advantage leads to VICTORY! So, if there's a Warwick at, say, 30% hp running to your side to grab a quick blue buff, call in a gank on that cheeky bastard, and profit from the fool's overextension.

Team Fights:
The most important part of this guide. Remember, you are the tank. Your job is to charge straight into the fight, initiate it, and go, "HELL NO BEEYATCH! NO SQUISHY CARRY FOR YOU! I AM THE ******* JUGGERNAUT, AND YOU ARE FOCUSING ME!" Now, some of you may be confuzzled at how to initiate team fights. Here's the standard combo: E > W > R > (delay) > Q
And then you just make damn sure that they focus you by rooting/slowing anyone that tries to focus your carries/squishies by making their life miserable through spam of all your abilities. Also, DO NOT RELEASE YOUR ULT UNTIL THE ENEMY TEAM STARTS RUNNING! I cannot stress the importance of this enough. I have seen far too many fights go wrong simply due to incorrect use of Vengeful Maelstrom. Remember, the more is absorbed by it, the stronger the release is. (Wait, that didn't sound nearly as bad in my head.)
Anyways, moving on from my mini-rant... If any enemies attempt to flee, you can do one of three things:
1) (Very Low HP) Predict their path, and toss a Sapling so that you can get both the impact/detonation damage. That should take care of them.

2) (Low HP) Twisted Advance. That should be all that's necessary for your team to mop him/her up.

3) (Moderate/High HP) Sapling Toss, Twisted Advance, and Arcane Smash them in the opposite direction of their base right after they get out of your root. Follow-up with another Sapling Toss if necessary.

Ganking:
Maokai's ganking capabilities are very good, as he has 2 CCs, a root and a slow/disable (if used correctly). His standard burst combo is E > W > (delay) > Q. If they've got a dash/warp/flash/jump, then go W > E > Q instead. It'll prevent them from doing jack for a whole 2.5 seconds, which is usually enough to wipe most squishies, or at least burn them badly enough to force a blue pill (recall).

Laning: Stay back far enough to avoid getting burned hard (against ranged anti-tanks) but close enough to last-hit with AA. An alternative is to pick up a blue buff and spam Saplings/Arcane Smash. At rank 5, these two abilities with NO AP items/runes can wipe out an entire creep wave. Make sure to constantly harass with E, which will wear down the enemy or zone them out. If they extend past their tower (the first line of brush off to the sides), and you know there aren't any enemy champs coming to gank you, pull off a burst combo and GTFO before they have a chance to respond. It goes E > W > Q. Why use Sapling before Twisted Advance? Because it guarantees the impact/detonation damage, and it removes the casting time for it, which gives you 1-2 AAs on the enemy.

This is the sandbag of the team. The bushwhacker, the one that forages ahead and takes all the damage. Expect to have a low amount of kills and a hella lot assists. When you choose this role, you are thinking team-play. You WILL fail if you do not team properly. However, if your team knows what they're doing, you will win. Guaranteed.

This is a little extra section explaining why this build works.
For one, you're going to have quite a strong laning phase, where you won't have to blue pill out more than once due to your Catalyst giving you a boost in health and mana every level. Now, it isn't just the Catalyst that's keeping you in lane, mind you. Mao's passive, Sap Magic, is the real clincher when it comes to sustainability. For every 5 spells that are cast in range of Maokai, he gains a Sap Ready buff which will heal him for 7% of his max hp on his next autoattack. The range on the spell cast is decent, too. (1200, I believe. This means that any spells cast within the majority of your area of sight will proc your Sap Magic.) That means that, between the harrassing with E, bursting/initiating with W and Q, and damage soaking that you're doing in general, you'll be healing up 7% of your MAX hp on autoattack.
Isn't that just effing awesome?
Allow me to give you a scenario: You're laning with a Kat against a Tryn and Rumble. That means ALL FOUR OF YOU HAVE AN INFINITE SOURCE OF SPELLS THAT FUNCTION ON A NON-MANA BASED SYSTEM!!! Which means that you can spam all day long, and every 5 casts, BOOM and you're up 7% hp. You can understand why this would make Mao an immovable object in lane, yes? :)
Now, enough raving about Sap Magic...
With your full build, at level 18, you're going to have about 4k health. Now, since almost no matches last that long, I'm going to be realistic and say you've got about 3k health. MATHS INCOMING!!!
3000/100 = 30
30 * 7 = 210
So you're gonna be healing back 210 hp EVERY 5 SPELLS CAST! In a teamfight, spells are gonna be dropping faster than meatballs from the sky. REMEMBER TO AUTOATTACK! Every damn second, other wise those yummy spells are going to waste ;)

5/31/2011: Changed masteries to 0/21/9 and respeced them, changed recommended summoner spells to Fortify/Clarity, fixed "Summoner Spells" section.
7/23/2011: Updated runes, now quints are hp instead of armor for early game presence and more healing off of Mao's passive.