Welcome, to my build on Kog'Maw, The Mouth of the Abyss.
Since this is my first build, please tell me if you think it's good or not after testing it and such. Thank you and enjoy.

This build is focused on AP and Attack Speed, if you didn't already notice ......

Pros:
Does percentage damage
Rips tanks apart
Considerably longer range than other champions when using W
Very heavy dealing ultimate that has a low cooldown
Armor reduction and passive attack speed

Cons:
Squishy, and I literally mean, Squishy.
Almost impossible to escape a gank
Extremly prone to CC

Greater Glyph of Force: AP. More AP!
Greater Seal of Force: Still more AP.
Greater Mark of Force: Even more AP?
Greater Quintessence of Force: Still, more AP.

No flat runes, we're focusing on how powerful we are late game with all of our items, which means per level runes.

The typical 9/0/21 build for AP champions.
Offense:
Archmage's Savvy: 3/3. More AP per level.
Cripple: 1/1. Adds armor and magic resistance reduction to target, and increases duration.
Sorcery: 4/4. Cooldown reduction.
Archaic Knowledge: 1/1. Magic penetration.


Utility:
Perseverance: 3/3. More health and mana regen. Useful.
Good Hands: 1/3. Death reduction. Less death timer = more game time.
Expanded Mind: 4/4. Increased mana pool.
Awareness: 2/4. More exp = Faster levels.
Meditation: 3/3. More mana per 5.
Quickness: 3/3. Makes you a bit faster.
Blink of an Eye: 1/1. Cooldown reduction on Flash.
Intelligence: 3/3. Cooldown reduction on skills.
Presence of the Master: Cooldown reduction on all summoner skills. Very useful.

Berserkers Grieves: +Movement speed and attack speed.
Madred's Bloodrazor: +Attack Damage, Attack speed, some armor. The passive on this is what makes you OP. It is what makes tanks and squishies alike fear you. It gives you an extra 4% magic damage each time you hit them with a basic attack.
Guinsoo's Rageblade: +Attack damage and ability power. The passive adds more attack speed and more ability power. Very useful.
Malady: +Attack speed and ability power. The passive causes the enemy to have even less magic resistance, further increasing how deadly your damage is, and also helping your teams nukes with the magic resistance reduction.
Nashor's Tooth: +Attack speed and ability power. Some mana regen and 25% cooldown reduction.
Rabadon's Deathcap: +AP. This last item will boost your damage by an enormous amount.

Try and max out your bio-arcane barrage first as that will give you your amazing percentile damage while also increasing your range. Use it whenever a fight starts or if you feel like harassing the enemy team. Next max out your caustic spittle. It provides you with extra attack speed and when you use it on an enemy it reduces their armor. Whenever you're going one on one don't be afraid to use this since the passive does not go away when you use it. Last skill you want to max out is void ooze since you won't be using it much early game, but it is useful late game when you have a bunch of AP and it really does some damage and slows the enemy down considerably. Max out your ultimate whenever you get the chance as with any champion.

When in a fight, choose your target, then proceed to use your Q on that person if you can, and then activate your W, and start firing away. Use your ultimate when you think you can get a kill or if you want to scout some bushes with it. Use your E to slow down enemies that are running away or coming towards you. You can also use your W behind a wall in team fights that are lucky enough to happen in the jungle, so you can act like a sniper and shoot down those pesky melee carries and the annoying nukes.

Exhaust: Slows down the enemy champion considerably, reduces their damage, and reduces their armor and magic resistance. Use on anybody you think you can kill, that is either running away, or trying to kill you. If you need to use it defensively, use it on said champion, use void ooze if possible, and start running away as fast as your little legs can take you.
Flash: Multi-purpose spell. You can use it to catch up to a running champion to get the last hit, or you can use to defensively and run away from whatever you're running away from. In jungles attempt to use it to flash over a wall if possible, then void ooze whatever path they have towards you, or if they're close use void ooze where you're heading, and start running.

No, you will not survive in a team fight if you go in head first. If you want to survive any team fight with this build, you have to wait for the fight to start for a few seconds, and when your enemies have found a general focus on who they're going to attack, come in and start tearing people apart. You have a pretty long range after all. It shouldn't be hard.

Remember to also not just stay in one spot and keep shooting. They'll eventually catch up to where you are and start killing you.

This is the end of my little build. I hope you enjoyed it and tested it out. If you are wondering why this is called "3 Second Death Kog'Maw", if you do the math you should be able to kill just about any champion in percentile damage in 3 seconds, since each of your hits do about 14% damage with your W activated. And now, with the patch buff, you can kill multiple enemies with the single use of your W.

ENJOY, VOTE, AND COMMENT.

Updated 8-31-11: Added additional notes and the last sentence of the summary.