I'm sure many of you are aware that the current convention in League of Legends is to send a carry and a support champion to bottom lane. Originally, these support champions were characters like Soraka or Sona, who could babysit their carry through any problem. Eventually, variety was introduced in the widespread emergence of more tanky supports who still had a heal, such as Alistar or Taric. Many of the more traditional supports started to build more tankily as well. Much more recently, very tanky characters such as Leona have begun to be used for their CC, soaking potential and mobility. What this breed of support lacks in sustain it makes up for in brutal zoning, burst damage, and pulling enemies out of position. In this guide, I will put forth my theory that Poppy is one of these most irksome of supports.
As with all things, I welcome constructive criticism or opposing arguments. Saying, however, something along the lines of, "No Phantom Dancer? -1" will do very little to help this guide.

There are benefits and negatives to using Poppy as a support.
PROS:
-Potent stun
-Burst damage at low levels
-Huge mobility
-Dominant in lane
-Wonderful initiator; Diplomatic Immunity means 8 seconds of invulnerability (to turret shots, too)
CONS:
-Damage peters off unless you get magic penetration
-No natural heal

Normally, I do not advocate staggering your abilities, but you'll understand why I do so on Poppy in a second.
Of course, Poppy's first priority as far as abilities go is her ultimate, Diplomatic Immunity. You'll want to get this at levels 6, 11, and 16.
Of Poppy's burst damage and lane bullying arsenal, her most important piece is her charge, Heroic Charge. We level this up on a high priority because doing so gives us a lower cooldown and significantly higher burst damage.
Now, Poppy's other two abilites, Paragon of Demacia and Devastating Blow, are less vital to level up. The biggest value of Devastating Blow is that it deals bonus damage based on the target's total health; the extra 20 damage you get on leveling it up doesn't increase its value significantly, especially when compared to Paragon of Demacia's bonus AD, armor and *gasp* percentage mobility. However, this bonus damage has a maximum threshold, which means that as the target gains more total health you have to level up Devastating Blow accordingly to gain the full 8% damage. Here is a table with the maximum health values of you target at which you'll want to level up your Devastating Blow:
Rank 1: 975
Rank 2: 1875
Rank 3: 2812
Rank 4: 3750
Rank 5: 4688
As you can see, almost all of the time, you'll only need a Devastating Blow at rank 3 to deal maximum damage to a very fed carry. But if an Amumu is wrecking your team, suddenly Devastating Blow becomes more valuable to level up. Level your Devastating Blow to match the target of your Diplomatic Immunity.

For Marks, Support Poppy doesn't really like anything. I choose to use Greater Mark of Magic Penetration for her level 2 burst combo, but it's really your choice. If you have a different Mark you like to use, feel free to comment on it! (Note: Devastating Blow turns Poppy's next attack into magic damage, so Greater Mark of Desolation does not apply to it.)
For Seals, choices are much easier. I personally prefer Greater Seal of Armor, because the majority of low-level fight damage is auto-attack damage, and in this build Poppy builds quite a bit of armor by late-game (so you don't need scaling armor).
For Glyphs, I use Greater Glyph of Scaling Magic Resist for the extra tankiness. Who doesn't love tankiness?
For Quintessences, I use Greater Quintessence of Movement Speed. Poppy is extremely positioning reliant because her stun requires a wall. A little boost that makes life easier. Recently, for fun, I've been running Greater Quintessence of Gold. I know it doesn't seem like much, but when you aren't farming +3GP10 is a nice plus to your income. Plus, I love my items, so it's indulging in something I find fun.
Of course, I welcome critique and alternative ideas. Please feel free to comment!

For the same reason that we chose Greater Quintessence of Movement Speed, we use Ghost at all times when playing Poppy. Ridiculous mobility is, in addition to being a huge asset, a lot of the fun of playing her.
There are two ways to bend Poppy in this build. The way I prefer is a more traditional support style, with the spell Heal and points in Utility to make sure you can actually heal in the clutch moments that the spell is necessary. Reasons for picking this style are:
-Level 1 Regrowth Pendant ( Wealth )
- Heal
-Better income ( Greed )
The other way to build Poppy forgoes tradition in supporting and takes on the tanky role at an earlier level through the Defensive tree. A Mastery build might look something like this:

W00T
1. Vayne: A proficient Vayne who levels Condemn is the best ever. Your laning opposition will be zoned away from any wall or bush or be chain stunned for massive damage.
2. Tristana: Though not as nice as Vayne, a good Tristana can set up a stun with her Buster Shot and has burst to match yours. An underused carry.
3. Caitlyn: Better range than Vayne and similar zoning, but less burst damage. A fantastic carry for Poppy to own lane with.
MEH
4. Miss Fortune: While not the hardest carry, you can help her get off some crazy Bullet Time. She's pretty tanky for a carry, too, which is awesome because you offer little to no sustain.
5. Ashe: An easy carry to run with. Range is nice, and so is perpetual slow. The poke in this lane is ridiculous. Make sure she focuses on farming while you move around. This is not a kill lane.
6. Sivir: Decent. The movespeed bonus is fantastic for you late-game, but there's no real synergy here.
BLEH
7. Kog'Maw: A fantastic carry, but you don't provide sustain or an attack speed bonus. He's going to struggle without the right babysitter.
8. Ezreal: Send him mid. Seriously. He farms great on his own; you're just stealing XP.
9. Urgot: This requires a mana battery like Soraka to be worth it. Pass.

STARTING ITEMS
sight ward is the preferred set of items for the Utility build. Between this and Valiant Fighter, you shouldn't die in lane.
If you're going Defensive masteries, you're making your job a little harder.
sight ward is what you'll have to go to have a Sight Ward at level 1.
CORE ITEMS
philosopher's stone kage's lucky pick
Many Poppies I have played with stack damage, citing their role as an assassin. However, much of their early fear factor comes from the percentage on their Devastating Blow and their stun's utility. Poppy is not very reliant on her items in the early-game, so good gold per 10 items will make up for lost farm and the tank aura on Aegis of the Legion will give those around her an additional benefit of her presence (totally optional). If the opposing team does not have significant or mitigate-able CC, feel free to switch out Mercury's Treads for Sorcerer's Shoes, to be sure you're hitting with the full force of your Heroic Charge and Devastating Blow. Any other early-game damage item is (almost always) just extra.
LATE-GAME ITEMS
Almost all of these items give Poppy a mobility boost over her target. Others increase her magic penetration.
is an item I feel is undervalued for Poppy. The CDR and health are both welcomed, but the speed boost is really ridiculous.
is normally what we'll build our Heart of Gold into. The armor and health don't hurt, and the regen aura and AoE shield allow both for your carry's continued sustain in lane and a clutch save (to make up for your lack of, say, Janna's shield).
is an important part of Poppy's burst combo if tanky items are not needed. Remember, you'll go from laning as a support to becoming a tanky initiator; you are no Sona. Generally, I only buy two gp10 items.
gives Poppy a lot of tankiness and a mobility boost over her surrounding enemies. This is the other option for using your gp10 item. Wonderful for taking down carries, and taking turret shots once Diplomatic Immunity wears off.
gives Poppy lots of tankiness and makes sure you can't run away from her Heroic Charge. Unlike many characters who pick this up, Poppy makes relatively minimal use of the slow and a lot of use of the tankiness.
is an alternative to the Frozen Mallet, as they both have the Phage effect. The Frozen Mallet applies a consistent slow (but don't get too excited about this), while the Trinity Force gives a consistent speed buff. The Frozen Mallet gives 700 health, while the Trinity Force gives burst on Poppy's Devastating Blow. Both are viable, and both are prohibitively expensive.
is a cheaper alternative to the Frozen Mallet/ Trinity Force debate. The 15% CDR and movement speed active are evocative of Shurelya's Battlesong. I usually don't pick it, though, because the armor penetration and attack speed largely go to waste on this build.
Note: Most people choose Trinity Force, because that is what they are used to. There is nothing wrong with that. Be aware, however, that you won't be farming very much as a support; I advise building it after other, more vital items (Like Locket of the Iron Solari). No one's arguing it isn't a fantastic item on Poppy, it's just expensive.
is a beautiful item. Tons of magic resistance AND a nice chunk of mobility. The health regeneration is the icing on the cake. Pick this up if any mages are giving you trouble.
is an item I've found myself taking more and more as an initiator. After my eight seconds of extreme concentration are over, my Poppy can be far behind enemy lines and easy pickings by an(other) assassin. Besides the resistances helping you tank, the anti-gimp helps when this sort of thing occurs.
helps with burst. This item should be taken when the opposing team has a really annoying tank that won't die, and your 8% max health damage will be significant. If the opposing team is heavy on magic damage rather than physical (as well as being very squishy), feel free to replace Void Staff with Abyssal Mask. "But why use a magic resistance-reducing item when the target has low magic resistance?" you might ask. The answer is that magic resistance has a diminishing return of effectiveness, which means that going from 30 magic resistance to 40 magic resistance has a much larger effect than going from 100 magic resistance to 110 magic resistance. This means that flat magic penetration and resistance reduction actually become more valuable as the target loses magic resistance. Percentage-based penetration, however, continues to be valuable at high magic resistance levels.

NOTE
Feel free to transition to a more assassin-esque Poppy late-game, rather than a traditional support. This build is primarily a method of viability for Poppy during the laning phase of a game, where normally there isn't a lane for an assassin. If your team needs more damage (eg., a carry on their team NEEDS to be killed), then Berserker's Greaves and an Infinity Edge might be what you need. Support your carry in the early game, and contribute how you'd like by the end.

Thank you for reading my guide! I hope you enjoyed yourself, and I hope my idea can be expanded on by the Mobafire community!

Best wishes,
-PranzatelliThanks to JhoiJhoi for the awesome banner!

This guide inspired my change from traditional tanky support all the game to a support that metamorphoses into an assassin: Khazem's got Poppy's role down pat.