Welcome to my first MobaFire build.
LeBlanc is by far the most fun and rewarding champs I have ever played so I thought I would make a build.
This build focus on high magic penetration.

Please read before you rate

Every city has its dark side, even one whose reputation is already of a questionable hue. Noxus - though its name is already invoked with a mixture of reverence and revulsion - is no exception to this simple truth. Deep within the winding dungeons that honeycomb the earth beneath its dark, meandering streets lies the real underbelly of this sprawling metropolis, a haven for all manner of malevolence. Amongst the cults, covens, and secret societies that call this labyrinth their home, LeBlanc, the Deceiver, presides over the Black Rose, a remnant from a lost, yet similarly unscrupulous time in Noxian history. Ruthless and seemingly ageless, LeBlanc and her ilk were a mainstay in Noxian political affairs during the era before the militarization of the Noxian government. In those days, this guild of powerful magicians met in secret to further their hidden agenda, and to hone a craft more subtle than that preferred by those currently in power.

While their exact motives have always remained mysterious, it was widely believed that the Black Rose was the true power behind the throne while the aristocracy still reigned in Noxus. When raw martial prowess became the ultimate determination of whose will held sway in the Empire, the Black Rose seemed to vanish overnight. Many believed that perhaps their time had simply passed, and that its members had put aside their quests for social and political dominance. When LeBlanc reemerged at the gates of the Institute of War, however, it became clear that these masters of shadow and flame had simply been biding their time, waiting for a new global authority to emerge: the League of Legends.

"The world is very different for those who cannot see beyond what is placed before their very eyes." �- LeBlanc, the Deceiver

Marks penetration is what makes LeBlanc a killing machine even against tanks the whole game.
Seals LeBlanc is a mana hungry champ early and these solves that problem quite good.
Glyphs These along with Archiac Knowledge and a fully stacked ((Mejay's Soulstealer)) makes her able to spam her skills alot more.
Quintessences Is just overall good for casters.

Pros
Lovely burst
Great assasin
Not ganked easily
Silenc
Cons
Squishy
Can be mana hungry if you spam
Requires fast hands for optimal harrasment

Mirror Image
When LeBlanc is brought below 40% health, she instantly becomes stealthed for half a second. When the stealth fades, she creates a Mirror Image that deals no damage and lasts for up to 8 seconds. This can only occur once per minute.

A Great Passive for escaping scouting or confusing the enemie team. Have laughed hard when a Mordekaiser have used his ult on my clone.

Sigil of Silence Sigil of Silence
LeBlanc projects an orb towards her target, dealing 70 / 110 / 150 / 190 / 230 (+60% of ability power) magic damage and marking the target for 3.5 seconds. If the target takes damage from one of Leblanc's abilities, the mark will trigger, dealing 20 / 40 / 60 / 80 / 100 (+30% of ability power) magic damage and silencing the target for 2 seconds.
Cooldown 6 seconds | Cost 70 / 75 / 80 / 85 / 90 mana | Range 700

A very nice nuke and silencer if used with another ability

Distortion
LeBlanc rapidly moves to a target location, dealing 85 / 125 / 165 / 205 / 245 (+60% of ability power) magic damage to nearby units. In the following 3 seconds, she can activate Distortion again to return to her starting location.
Cooldown 20 / 18 / 16 / 14 / 12 seconds | Cost 80 / 90 / 100 / 110 / 120 mana | Range 600

Lovely chase/escape ability especially with her ult. Early game Sigil of Silence and this is great for poking or killing. It is also a great farming tool with enough AP

Ethereal Chains
LeBlanc flings illusionary chains towards a target location. If it hits an enemy unit, it will deal 40 / 65 / 90 / 115 / 140 (+40% of ability power) initial magic damage and slow their movement speed by 25%. If the target remains shackled for 2 seconds, the target takes additional 40 / 65 / 90 / 115 / 140 (+40% of ability power) magic damage and is unable to move.
Cooldown 10 seconds | Cost 80 mana | Range 950

Good CC skill for saving or killing.

Mimic
LeBlanc can cast the previous spell she cast. The mimicked spell deals 10% / 25% / 40% increased damage.
Cooldown 40 / 32 / 24 seconds | Cost 100 / 50 / 0 mana |

Great ult with low CD that makes you 2 hit the squishy targets late game and take a good chunk of HP from everyone else.

I take a point in Sigil of Silence at level 1 for poking
then I take for the escape and silence.
level 3 I take another point in Sigil of Silence for more dmg and in level 4 i grab .
I first max Sigil of Silence then and maxing whenever I can

If I am in lane I might take first then Sigil of Silence and then I just follow my normal sequence.

Summoner Spells to get.

Good escape or chase if your abilities are on cooldown
To pick up the easy kills when you have used your combos.

Other good spells

Good against CC
Extra escape method
Great for early mana starving
If you are constantly targeted it is nice with a little heal

Spells not to get

Not really of much use
More of a tank spell
Really?
Not really usefull for LeBlanc your DPS should have this anyways

Core Items

These are the items you get no matter what.

If you are doing good early on grap Mejay's Soulstealer
and later on you sell your and for something else depending on the situation.

For Heavy dmg take a as it works great with her Q-R combo or to get through more MR and give youself some aswell.

For Survival take a for the HP and passive or for the armor and passive

Solo mid

Early game-lvl 1-6

You just wanna try to use your Q on the enemie and then W for the dmg and silence with your E for the stun afterwards. when you hit lvl 6 you can perform your full combo you can either W-Q-R-E or Q-R-W-E and if that don't kill them throw an ignite and your Q again for the kill.
If you are doing well you port back for Mejay's Soulstealer if not you grap your shoes.

Mid game-lvl 7-12

Now your goal is to gank or keep killing in mid. If you choose to gank your mission is to stay out of sight until they make an aggresive move. Then you strike. W-Q-R-E should finish them with your teams help.

Late game-lvl 13-18

Now you are the killing machine that can 2 hit the squishies and scare the tanks so why not just use it and kill them with your team for the last push if they haven't already made the surrender vote. If you have all your items I suggest selling the for one of the alternative items depending on the situation.

Lane

Early game-lvl 1-6

Lane can be better for LeBlanc if you have a good stun slow or heavy dps champ with you. If you do you can take first for an easy firstblood ex you and Sion with are waiting in the brush and then the enemy Kog'Maw walks in. You throw on him with after the stun is over Sion cast his and if necessary and your team have the firstblood.

Mid game-lvl 7-12

Now you and your teammate will have some kills and you can start ganking just remember you are not an initiater you are an assasin so wait until the enemy have pushed your minions back let a tank or fighter attack and then jump out of the brush and kill them with your teams help.

Late game-lvl 13-18

The lanes should be broken by now so just run around and assasinate and push where you can just remember to stay behind your team and let them initiate.

Most irritating champs to lane against

Swain: and are so annoying and can keep you away effectively + when he hit level 6 you can't really burst him down without because of his ulti

Vladimir: and nothing more need to be said.

Sivir: Spell shield

Nocturne: Spell shield and Fear

Morgana: Spell shield

Cho'Gath: Silence

Kassadin: Silence

Malzahar: Silence and

Zilean: Bombs and ulti

LeBlanc: SilenceSigil of Silence Stun ChaseDistotion

How to play against will be added shortly.

That's it please rate it and give me feedeback so I can improve the build
PS: English is not my main language so sorry for bad spelling

Updates:

April-2-2011: Build published.
April-6-2011: Added history. More details on how to play in lane and annoying champs to lane against. Updated items.