Is it possible in practice to reduce someone's magic resist / armor below 0?

I've seen a lot of penetration items but not too much items with reduction. Since you can't penetrate below 0 how often it occurs (and how) that you reduce someone's magic resist / armor below 0? With which champions?

It is possible to get someone's magic resist/armor below zero with proper items/skills/spells. Like you said magic/armor penetration doesn't do that. Only % magic/ resist/armor reduction and flat magic resist/armor reduction.

MAGIC RESISTANCE REDUCTION

ITEMS

• Abyssal Scepter: Unique Aura: Reduces the magic resistance of nearby enemy champions by 20. +70 ability power, +45 magic resistance. 2650 gold.
• Malady: Unique Passive: Your basic attacks reduce enemy magic resistance by 4 for 8 seconds. Stacks up to 7 times. +25 ability power, 50% attack speed. Unique Passive: Your physical attacks shred your target doing 15 + 10% of your ability power as bonus magic damage. 2035 gold.

ABILITIES

Flat magic reduction:

• Amumu's Cursed Touch passively makes his autoattacks reduce the magic resistance of enemies hit by 15 / 25 / 35 for 3 seconds.
• Fiddlesticks's Dread passively reduces the magic resistance of nearby enemies by 10.
• Heimerdinger's H-28G Evolution Turret, at rank 2 and above, reduces armor and magic resistance by 1 per hit for 2 seconds (effect stacks up to 50 times).
• Kog'Maw's Caustic Spittle reduces the magic resistance of the target by 5 / 10 / 15 / 20 / 25 for 4 seconds.
• Morgana's Tormented Soil reduces the magic resistance of all targets by 4 / 5 / 6 / 7 / 8 each second for 5 seconds.
• Ryze's Spell Flux reduces the magic resistance of all targets by 12 / 15 / 18 / 21 / 24 magic resistance reduction for 5 seconds.
• Soraka's Starcall reduces the magic resistance of all targets by 8 / 9 / 10 / 11 / 12 for 8 seconds (effect stacks up to 10 times).

Percentage magic reduction:

• Trundle's Agony steals magic resistance of the target by 15% / 20% / 25%, this amount is doubled over the next 6 seconds.

• Jayce's Transform: Mercury Cannon grants his next auto attack to reduce the enemy's magic resist by 10% / 15% / 20% / 25% for 5 seconds.

• Karthus's Wall of Pain reduces the magic resistance of all targets affected by 15% for 5 seconds.

• Kayle's Holy Fervor allows her autoattacks against enemy champions reduces their magic resistance by 3% for 5 seconds (effect stacks up to 5 times).

• Nidalee's Bushwhack reduces the armor and magic resistance of enemies that step on the trap by 20% / 25% / 30% / 35% / 40% for 12 seconds.

SUMMONER SPELLS

• Exhaust, when augmented by Summoner's Wrath, reduces the target's magic resistance by 10 for its duration.

ARMOR REDUCTION

ITEMS

• The Black Cleaver: Unique passive: Dealing physical damage to an enemy champion reduces their Armor by 6.25% for 4 seconds. This debuff stacks up to 4 times.+250 health, +50 attack damage, +10% Cooldown Reduction. Unique: +10 Armor Penetration. 3000 gold.

ABILITIES

Percentage Reduction:

• Jarvan IV's Dragon Strike reduces the armor of all enemies hit by 10% / 14% / 18% / 22% / 26% for 3 seconds.
• Jayce's Transform: Mercury Cannon reduces the armor and magic resistance of the enemy hit by his first attack after cast, by 10% / 15% / 20% / 25% for 5 seconds
• Kayle's Holy Fervor reduces the armor and magic resistance of enemies hit by her autoattacks by 3% for 5 seconds. This debuff stacks up to 5 times.
• Nidalee's Bushwhack reduces the armor and magic resistance of enemies that step on the trap by 20% / 25% / 30% / 35% / 40% for 12 seconds.
• Renekton's Slice and Dice, when empowered by having more than 50 Fury, Dice will reduce the armor of all enemies hit by 15% / 20% / 25% / 30% / 35% for 4 seconds.
• Shyvana's Flame Breath reduces the armor of all targets hit by 15% for 4 seconds.
• Trundle's Agony reduces the armor and magic resistance of his target by 15% / 20% / 25% and gains for the same amount. This amount is doubled over the next 6 seconds.
• Urgot's Noxian Corrosive Charge reduced the armor of all enemies hit by 12% / 14% / 16% / 18% / 20% for 5 seconds.
• Vi's Denting Blows reduces the target's armor by 20% for 4 seconds after every third consecutive hit.
• Wukong's Crushing Blow reduces the armor of the target hit by 30% for 3 seconds.
• Xin Zhao's Challenge reduces the armor of targets hit by his basic attacks or Audacious Charge by 15% for 3 seconds. Only one target may be challenged at a given time

Flat Reduction:

• Corki's Gatling Gun fires automatically every half second, reducing enemies armor with each hit by 2 / 4 / 6 / 8 / 10 for 2 seconds.

• Heimerdinger's H-28G Evolution Turret, while in ranks 2, 3, 4 and 5, the turrets' attacks reduce armor and magic resist by 1 per hit stacking up to 50 times.

• Kog'Maw's Caustic Spittle reduces the target's armor and magic resistance by 5 / 10 / 15 / 20 / 25 for 4 seconds.

• Nasus's Spirit Fire reduces the armor of all enemies in the area by 20 / 25 / 30 / 35 / 40 for the time they are on it plus 1 second.

• Rammus's Puncturing Taunt reduces the target's armor by 10 / 15 / 20 / 25 / 30 for 1 / 1.5 / 2 / 2.5 / 3 seconds.

• Taric's Shatter reduces the armor of all enemies hit by its activation damage by 10 / 15 / 20 / 25 / 30 for 4 seconds.

SUMMONER SPELLS

• Exhaust, when augmented by Summoner's Wrath, reduces the target's armor by 10 for its duration.

SOURCE

LOL WIKI

EXAMPLE

An easy example that everyone can try out and find out if armor can actually go negative is this:

Heimerdinger's level 1 Armor is 10. So get Corki and get exhaust with the Summoner's Wrath mastery.
Go meet each other at a lane (level 1) and exhaust him and use Gatling Gun on him for 2 seconds and watch his Armor become 10 - 10 - 8 = -8!!