I'm not too good at english, it isn't my native language, so i'm really sorry for all typos and things like that!
Things i'm currently working on will be listed in first comment under this guide!I love Teemo.
Global taunt is OP.
Wards = win = mushrooms
Support + AD carry < Teemo + AD Carry (blind OP)
Just sayin'
Also, please, read whole guide before voting.

If you will downvote i don't care, even if this will be most downvoted guide on mobafire. I just wanted to share my idea.
Support Teemo is funny, kind of viable, and can harras that Vayne to da ground.

Just some copy-paste from my message to ThaKinetic.





Or, if u don't have time to read anything:

Good harras.
Can disable AD Carry, thus making 2v2 a 2v1,5 (and that's 1,5 only if enemy AD carry has some useful abilities)
Stealth (that means he can trap enemy support and kill him 1v1)
Can be harrased to ground, forced to recall, bursted down in a second.

spaaaaaaaaaaaaaace

Pros Long lasting blind with good range. Ghost + Move Quick = HAHAAHAH HE'S SO FAST LOL (i personally recommend Cottontail Teemo for that.) His passive can be used to make succesful traps. Very annoying with his blind and poisons. Mushrooms can be used in teamfights to cut off escapes, slow, deal tons of damage. Can ward multiple places without buying wards. Can disable enemy AD carry for long time using blind and exhaust.

space

Camouflage

space

Cons SQUISHY AS HELL Gets focused alot.(a.k.a. global taunt) If u're spamming your abilities, u will run out of mana fast. Short range, u need to continuously attack target to deal good damage. Not counting items, no sustain whatsoever.

I'm going for alot of gold/10 sec, some magic penetration and CD Reduction.
By this i mean 9x Greater Seal of Gold, Greater Glyph of Cooldown Reduction, Greater Mark of Magic Penetration and 3x Greater Quintessence of Gold.
That, together with masteries, gives u around 7 gold/10 sec (that's 42 gold/minute, 420 gold/10 minutes and 840 gold/20 minutes - free Emblem of Valour.
It also gives u 9 magic penetration, so together with Sorcecer Shoes your mushrooms will penetrate about 30 magic resist, what gives u almost true damage (= tons of damage) vs ranged carries not building magic resist. Same with Blinding Dart, Wit's End and poison.
Glyphs are here just so you can spam Q and mushrooms.

U can exchange gold/10 seals for mana/5, but Teemo doesn't have that much mana problems unless u spam Q or spam mushrooms (like, u have 3 at once).

Recommended:

Disable. AD. Carry. That's all.

You're relying mainly on DoT's and on-hit effects, and ghost helps alot in applying these.
Don't get me wrong, Flash isn't bad, but you don't even have any stun or something, so i don't see why you would take Flash.

Viable:

Some fair damage. Choose it to counter Dr. Mundo, Vladimir, Mordekaiser etc.

I already wrote what i'm thinking about it.

If you don't have any healer*, sure, why not?

*By healers i also mean champions with in-built heal/lifesteal - Ezreal needs Heal more than Trundle or Nocturne.

Overwhelming map control, together with mushrooms.

You're support after all, aren't u?

First of all, i buy Doran's Blade first for survi and bit better damage output early game.
Item build i've listed on top of this guide is in my opinion
best for playing Teemo as a support. (not counting Rylai's)
Other than that, u can exchange Rylai's for some cooldown reduction, or Wit's End for Zhonya's if enemy doesn't have magic damage dealers.
I feel like i don't have to explain my item choices - Stark's is here to help your AD Carry and offtanks, Will of the Ancients will greatly help AP Carries and AD Casters (like Renekton) - most of AD carries also benefit from it.
Wit's End will help you in dealing constant damage to enemies, as well as boosting your survi.
Frozen Mallet is here only for slow and that huge chunk of HP.
As of boots, u can change them for Mercs or Berserks if you like them more - if you lack CD Reduction, buy Ionian Boots.

Thought only real reason to buy Rylai's Crystal Scepter is permaslow from Toxic Shot and alot of HP - if you don't need slow OR/AND HP, buy Nashor's Tooth!!!

Whole build costs about 15k gold, and that is much - point is, you don't need every item to be effective.

Will of the Ancients:

You can build or Rabadon, or Will of the Ancients on support Teemo (for more supporting mushroom power ofc)
So why WotA and why shouldn't you build Rabadons?
Will of the Ancients gives you 50 ability power + 30 ability power as aura, 80 ability power overall.
Aura ability power is also granted to your teammates, making it:
110 AP total if there's 1 teammate near you.
140 AP total if there are 2 teammates near you.
170 AP total if there are 3 teammates near you.
200 AP total if there are 4 teammates near you.
It also gives you and nearby teammates (1200 range) 25% spellvamp.
Rabadon doesn't give spellvamp, and alone it gives 184 ability power, so is almost surpassed by WoTA if there's 3 teammates near you.
With Rabadon's your mushrooms will hurt more, but that's it.
Will of the Ancients gives your AD/AP casters more sustainability in teamfights, allowing them to deal more damage, so overall WotA gives more damage than Rabadon in your hands.
Also, in my opinion, you should buy WotA even if your AP carry already bought one - it will help him anyway (making 25% spellvamp 50%), even thought you'll waste ~90 ap, as auras don't stack on teammates(they kinda do stack on owner thought, because owner always receives his own aura, and he also gains yours).
And WotA costs 2100 gold, while Rabadon costs 3600 gold.
You shouldn't even think about replacing Will of the Ancients, unless you have auto attackers only in your team.
As of bonus damage for youself:
Will of the Ancients gives you:
64 more Blinding Dart damage.
11,2 more Toxic Shot damage/second.
64 more Noxious Trap damage.

Stark's Fevor:

I personally love this item. It's like giving The Brutalizer, Vampiric Scepter and Eleisa's Miracle life regen to your entire team. Only problem with this item is your enemies can see Stark's Fevor debuff, even if you are invicible or in the bushes. Thought it doesn't reveal you, they just know that you're within 1200 range of them.
This item also shouldn't be replaced, as it and Will of the Ancients cause that you can be called "support", not to mention Emblem of Valor greatly helps you during lane stage.

Under construction.

Rabadon's Deathcap - selfish, but if you want alot of AP, replace Rylai's with it. Not recommended thought. I'll talk about it more in next chapter!
Zhonya's Hourglass - place mushrooms under your, shoot one, two times, use Zhonya's and let that Trynda die.
Oracle's Elixir - if enemy has Evelynn, Twitch, Teemo*, or your team wants you to destroy enemy wards!

* Or any other stealth champion.

Nashor's Tooth - Some attack speed, mana regen, ability power and cooldown reduction - everything Teemo needs. If you want it, you should replace Rylai's with it, but you should buy it after Emblem of Valor or after Zeke's Harbinger.
Malady - If you team is heavy on magic dmg, or you love this item - replace Wit's End or Rylai's with it.
Quicksilver Sash - dat Malzahar... dat Warwick... DAT Urgot...
I feel like this item is pretty self explanatory, isn't it?
Madred's Bloodrazor - if enemy has alot of champions stacking HP. Replace Wit's End, Frozen Mallet or Rylai's with it.

TO BE ADDED!

So, you're Support Teemo, eh? You're dealing reasonable, thought not too big damage.

Under construction.

camouflage It's my favourite passive in the whole game. It can be used to set up traps, hide from enemies, and it even makes u invicible when u're recalling! Who would not love this? Attack speed buff after revealing is great too.
Good harrasing tool, deals some good damage, disables AD carries and makes offtanks weaker. I like that skill very much. Problem is, it eats your mana very fast, and cooldown is kind of long.
Not too much about this ability. It gives u some movement speed. Great together with Ghost thought!
On-hit-damage-over-time effect, also permaslow from Rylai's. Not bad, not too got thought. Remember that early, mid-early game it can be denied by Health Potion.

Quoted:

OMG THAT Kennen RAN WITH 100 HP... Oh.. wait.. YES! GO INTO THESE BUSHES... YES YES HAHAHAHAAHHAHAHAAHAH /all AHAHAHAHAHAHAHAHAHAAH
Enemy has been slain.

Other than that, useful for warding, slowing, things like that. I love this skill, and later on i will add whole chapter about it!

Under construction.

Note: These aren't all locations where u can put mushrooms, remember that and don't put mushrooms just in places marked above!
Other than marked places, i usually place mushrooms in thin places, or during teamfights under/near my enemies.

Legend:

Purple Mushrooms - optional, depending on which lane are you, etc.

Red Mushrooms - very important mushrooms, you must have them here.

Orange Mushrooms - important, but if u're unlucky they will be useless.

(Note: There's one orange mushroom on top, but there is not on bot. This is because i see solo top ganked more often than bot)

Blue Mushrooms - purely defensive mushrooms, because of them i once managed to barely win teamfight with 2 inhibitors lost, defend and counterpush.

Green Mushrooms - last-stand mushrooms; just for slowing enemy down before he will reach nexus turrets.

White dots - Nexus turrets.

We max Blinding Dart first to disable carries for longer. Also, it's good harras in laning phase.
'til level 3 i like to have point it all three abilities.
Usually, Toxic Shot will be maxed second - this is unless u're facing good harraser and you will need to Move Quick(see what i did thier?) to dodge skillshots (ex. Urgot).
We pick Noxious Trap at usual levels - 6, 11 and 16.

mana masteries over reduced death time/faster movement because of Teemo's mana problems when u're placing multiple mushrooms at once, thought you can take movement speed/reduced death time if you took mana/5 runes, or don't use your abilities too often.
I pick experience masteries so i won't be underleveled when i'll leave lane to place some mushrooms.
I go for Mastermind to be able to use Exhaust and Ghost more often.
In offense tree u can take Demolitionist over Arcane Knowledge if you will take Teleport and push alot.
I don't know if Scout works on mushrooms thought.
Anyway, you should use Purple Wards sometimes to destroy their wards.

At the moment i have only 2 Teemo games in my match history, i'll update this when i will play some more games. Also, it's kinda easy to start feeding enemy team, as you are squishy AS HELL.
Just sayin'.

Maybe these ain't the best scores, but they're not so bad :).

Also, there's example of a game where my team dind't pick even one offtank and picked another support - i started with 2/0/0 score, but ended feeding, as we lost every single teamfight. While support Teemo CAN deal damage, if you will deal most damage in team*, you will most probably lose - just because your range is small, and you ulti usually can't change output of teamfight.

*You have smaller range than most of carries, no real escape and u're squishy. If there's noone dealing more damage than you, you will be focused and dead in one second.

And this one, sir, is win with support Teemo in RANKED! It works!

I'm sure people will hate me for this guide.
Anyway, it is my first guide, so if you have any suggestions, thoughts, share them with me.
If you will downvote, one Teemo will die. Poor, cute Teemo :<.

As of 07.01.2012, my guide is in top-10 Teemo guides!
Same day i also got 1000 views, still only few comments thought D:

To-do:

Anti-towerdive mushrooms section.
Many things.

Also, i'd love to thank jhoijhoi for Guide about making guides, and MrNicholas for saying that support Teemo is viable.