I am improving this guide 24/7 :D
*EDIT: You can downvote me, but at least give a reason to let me know what is wrong and where. :D

There exists an ancient order originating in the Ionian Isles dedicated to the preservation of balance. Order, chaos, light, darkness - all things must exist in perfect harmony for such is the way of the universe. This order is known as the Kinkou and it employs a triumvirate of shadow warriors to uphold its causes in the world. Akali is one of these shadow warriors, entrusted with the sacred duty of Pruning the Tree - eliminating those who threaten the equilibrium of Valoran.
A prodigious martial artist, Akali began training with her mother as soon as she could make a fist. Her mother's discipline was relentless and unforgiving, but predicated on the fundamental principle: "We do that which must be done." When the Kinkou inducted her into the order at the age of fourteen, she could slice a dangling chain with a chop of her hand. There was no question – she would succeed her mother as the Fist of Shadow. She has had to do much in this role which others might find morally questionable, but to her it is in service of her mother's inviolable doctrine. She now works with her fellows Shen and Kennen to enforce the balance of Valoran. This hallowed pursuit has unsurprisingly led the triumvirate to the Fields of Justice.

"The Fist of Shadow strikes from the cover of death itself. Do not impede the balance."

Twin Disciplines [Passive]
(Innate): Discipline of Force - Upon obtaining 19.5 ability power, Akali's auto attacks deal 10% bonus magic damage, increasing by 1% for every 5 ability power gained thereafter. (Magic damage doesn't affect towers, but it affects other structures.)
Discipline of Might - Upon obtaining 9.5 attack damage, Akali gains 10% spell vamp, increasing by an additional 1% for every 5 attack damage gained thereafter.

Mark of the Assassin [Q]
(Active): Akali throws her kama at a target enemy, dealing magic damage and marking the target for 6 seconds. Akali's melee attacks against a marked target will consume the mark dealing the same magic damage again and restoring energy.
Cost: 60 energy
Range: 600
Cooldown: 6 / 5.5 / 5 / 4.5 / 4 seconds
Magic Damage: 50 / 75 / 100 / 125 / 150 (+0.4 per ability power)

Energy restored: 20 / 25 / 30 / 35 / 40

Twilight Shroud [W]
(Active): Akali throws down a circular cover of smoke that lasts for 8 seconds. While inside the area, Akali gains armor and magic resistance and becomes stealthed. Attacking or using abilities will briefly reveal her. Enemies inside the smoke have their movement speed reduced.
Cooldown: 20 seconds
Range to center of AoE: 700
Radius of AoE: 300
Cost: 80 / 75 / 70 / 65 / 60 energy
Armor and Magic Resistance Bonus: 10 / 20 / 30 / 40 / 50

Slow: 14 / 18 / 22 / 26 / 30 %

Crescent Slash [E]
(Active): Akali flourishes her kamas, hitting nearby units for physical damage.
Cost: 60 energy
Radius of AoE: 325
Cooldown: 7 / 6 / 5 / 4 / 3 seconds
Physical Damage: 30 / 55 / 80 / 105 / 130 (+0.6 per attack damage AND +0.3 per ability power)

Shadow Dance [R]
(Active): Akali moves through the shadows to quickly appear next to her target and deal magic damage to it. Akali gains an Essence of Shadow once every few seconds up to a total of 3, affected by cooldown reduction. The time to gain a charge doesn't count when at maximum charges. Additionally, Akali gains an Essence of Shadow for a kill or an assist.
Cost: 1 Essence of Shadow
Range: 800
Cooldown: 2 / 1.5 / 1 seconds
New Essence of Shadow: 25 / 20 / 15 seconds

Magic Damage: 100 / 175 / 250 (+0.5 per ability power)

Skill Sequence: R > Q > E > W.
nothing more.

Usual combo to harass:
Q > wait till CD is off Q > R > auto attack > Q > E > auto attack > W > back off > ...

Usual combo to kill in 1v1:
Q > R > auto attack > E > * > Q > R > Auto attack > ** > ...
*[Use W here if necessary] **[Repeat same thing again if target is alive]

Usual combo in team fight:
R > W > E > Q > auto attack > ...

DO NOT ENGAGE FIRST AS AKALI, DO NOT INITIATE THE TEAM FIGHT, DO NOT DASH IN LIKE A ****...
or
"YOU HAVE BEEN SLAIN!" there you go monochrome TV for next 10 - 60 seconds.....

Mark.
Greater Mark of Magic Penetration EDIT: OR Greater Mark of Attack Damage x 7 Greater Mark of Magic Penetration x2
Seal.
Greater Seal of Health
Greater Mark of Evasion
Glyph.
Greater Glyph of Ability Power
Greater Mark of Scaling Ability Power
Quintessence.
Greater Quintessence of Ability Power
Greater Quintessence of Magic Penetration
Greater Quintessence of Scaling Ability Power
Greater Quintessence of Health

20 AP and 10 AD.
24 flat ap from runes can make it, start with a doran's blade gives you minor life steal, 100 hp and 10 damage to activate the other passive.
allowing you have some spell vamp , and extra magic damage on auto attacks.

ALTERNATIVE:
Start with a boot and 3 potions
require 7x Greater Mark of Attack Damage 2x Greater Mark of Magic Penetration

21/0/9 is the best for akali IMO.
akali is a anti-carry style melee dps assassin, she needs high burst and offensive summoner ability.

Good ones :D
70% auto attack reduction 35% spell and item damage reduction, 40% movement speed reduction. also, reducing 10 armor and MR, how nice!
Reducing healing! and true damage [ignores any armor/MR/dmg reduction buff] over time, how nice! [Good to take if you see Vladimir, Mundo, Fiddle sticks etc...]
Ignoring unit collision is everyone's dream.
Over walls, surprise ganks, dodging skillshots, and much much more.
But, for me, shroud she can escape easily therefore I don't use any escape summoner ability EXCEPT I know Lux, xin zhao, ryze, sion and taric is gonna be in same team D:

":/" ones :l
Cleanse is good againsts CC heavy team.
Lose less gold/exp, or save towers when you really need to shop/heal.

bad ones :(
Nonono, your ulti is not globle D:
lulz, not going to explain this.
Great defending ability that makes tower to a invulnerable dps splash machine, but you are not a support role or tank :D
Are you sona? soraka or taric? no? dont take it then :D
You got your spell vamp passive, why this?
Haha, if clarity replenish energy, i'd take this D:

[CORE]
Gives a insane amount of AP.
The more AP you have, the more damage output from your Q and auto attack.
Spell vamp + life steal FTW! Plus the AD gives more spell vamp as well from her passive.
Enemies like Garen easily get away from you? Buy this item. not to mention the 500 hp bonus.

[Optional]
If you are facing AD heavy team, buy it. Nice active and AP still.
Very cost-effective early game hybrid item and a nice passive.
Stacking MRs? Force of nature and banshee on that amumu? GET THIS AND ROFLSTOMP HIM.
Facing AP heavy team? buy this and own them.
If you are so confident that you will get stacks, or you got like 5 kills in 20 min, buy it.
Anti-CC boot with nice little boost of MR.
For extra Magic penetration.
This item is a cheap early game item gives some hp/ap and extra magic penetration.
If you really hate mercury's treads, buy this for the tenancy effect instead.

Pros :D
Instant kill enemy squishes!
Great escaping ability in bushes [If they don't have anti stealth]
Dash ability!
Amazing spell vamp with hextech gunblade [benefits from AD boosts as well]
Yellow mana! [Energy regen per 5 = 50 XD!]

Cons D:
Focused by enemy team in team fight = Instant kill!
Oracle destroys your stealth advantage
Globle taunt like Evelynn before stun > slow nerf and twitch, People like to focus you!

LUL This guide is not even finished, how can i summary it? :S

Change log:
[2011/07/24] Alternative Runes and passive active way added.

[2011/06/26] Added Haunting Guise, Moonflair SpellBlade to Optional items.

[2011/06/25] Akali - The Shadow Dancer Guide Starts here.