The board game NATO about a Soviet attack into Western Europe has rules for tactical nuclear weapons (5-15 kilotons). The rules for strategic nukes: Soak the map with lighter fluid and apply a flame.
This is a mechanic in Warhammer Fantasy Battles where someone who fully obliterates a foe in a challenge is rewarded with a higher combat resolution.
Invoked in Iron Claw which actually has a mechanic where dealing more damage to an enemy than it takes to kill them has an effect on game play. Specifically, nearby allies become afraid, there's not enough of the body left for any Necromancy spell that requires a body to work on them, and the game's only true resurrection spell can still revive them, but is unable to completely restore them from their severe mutilation and leaves them permanently disfigured.
In many card games, some players will wipe out the opponent’s entire defense, drain his lifepoints to bare minimal, and then summon a full fleet of powered up monsters to wipe out what little the opponent has left, despite the fact even one of them could wipe out the remaining life points 10x over. Many gamers refer to this strategy as an Onslaught.
"The Book of Vile Darkness" has a particularly vicious spell: Apocalypse from the Sky. The damage per square foot isn't stunning by the standards of 9th-level spells, but the blast radius is ten miles per caster level. According to the description, the spell ''typically levels forests, sends mountains tumbling, and wipes out entire populations of living creatures."
The cost of casting that is maybe even more overkill - casting it takes an artifact (high grade Unobtainium and likely better used another way) and damaging your own constitution to the extent of killing an average person (severely weakening tougher casters), does even more damage to the caster's wisdom (non-wisdom based casters without overly high wisdom scores are likely to make themselves helpless in an instant) and even damages your wisdom just by being prepared. And it takes an entire day to cast. Still, it has its uses. There is a spell (Sadism) that grants you a bonus on your die roll based on how much damage you inflicted last round. If you just wiped out an entire continent, you can basically succeed in any die roll. Actually, that roll is likely to be overkill too.
In Elder Evils (a book entirely dedicated to Eldritch Abominations), one such Evil, Atropus, the World Born Dead, can only appear / be summoned when a great amount of deaths occur in a catastrophe. One way that the book details that could happen can be by an attack of a demonic army upon the Material Plane. The other... is Apocalypse From the Sky.
There was a homebrewed Ultima spell (from the Final Fantasy games) 3.5 D&D. It did about all of those things and it required all of your spell slots to memorize it for the duration, had a bit of a time limit before it started damaging even more wisdom, and made you pass full out for like maybe half a month after use. Granted you could probably kill anything with it, barring maybe that one virtually-unkillable monster
And to go one better, a combination of metamagic when applied to the Locate City spell can do the same as (or better than) Apocalypse from the Sky. Locate City has a radius of ten miles per caster level. Apply a Snowcasting feat to add the Cold subtype, Flash Frost to make it deal 2 damage to everybody within the radius when you cast it, Energy Substitution to switch the damage to electric, Born of Three Thunders to allow a reflex save, and then Explosive Spell to force a reflex save or be blasted out of the area and taking 1d6 damage per ten feet traveled (which is, let us not forget, ten miles per caster level). Depending on DM adjudication, if this also applies to buildings, trees and other things within the radius of the spell, then you might just have a world-ender on your hands by environmental extinction. You may also need a feat from another sourcebook that turns a circle effect into a sphere. At 18th level, you can add Arcane Thesis, Widen Spell and Enlarge Spell to the combination, which should increase the area to a 400-mile radius. For comparison, England is only about 390 miles long. Kaboom indeed, and all before 20th level. With no negative effects beyond the book that the DM will probably be throwing at your skull.
And even that's playing it nice. After all, swap out the damage effects for fell-drain and instead of a crater with nothing left in it, you have 99% of what was in there dying of negative levels... and raising as an army of level draining wights the next day. add in widen and enlarge to that and, well, let's say all life on the planet is screwed...
Warhammer 40,000 lives off this. A fairly average pistol (a handheld fully automatic, armor piercing recoilless rocket-launcher) is an invocation of the Chunky Salsa Rule by anyone else's standards, and that's not even counting the standard lasgun of the Imperial Guard, which cleanly blows off the heads and arms of unarmored humans, or the pistols that fire shuriken or bugs that eat their way through your body to your brain in the brief few seconds that make up their lives, Oh and did we mention the flamethrower pistols that nuns in jet packs can wield akimbo?
Tau pulse rifles don't just kill Guardsmen. They kill Guardsmen very, very thoroughly. (Without cover, five hits in six are instantly lethal.) Pulse rifles are also the only basic ranged weapon except the Necron gauss flayer capable of putting the fear of the Tau'va into Space Marine Rhino vehicles and Ork looted wagons from the front. Railguns are even more spectacular, capable of reducing the largest and toughest tank (outside of Apocalypse) into a rapidly-expanding cloud of smoke and shredded metal.
Dawn of War brings this to the extreme within the game's system - no Earth-Shattering Kaboom, though. Granted, it starts off small with highly gruesome finishing moves between individual units, but reaching the top tier of almost any race brings wholesale death and destruction. The Necron's fully-awakened Monolith, for example, is a slowly-moving, teleporting, troop-producing, death-beam spewing glacier of epic proportions, and usually signals game over for anyone who receives it teleporting into the back of their one remaining base. Meanwhile, the Imperial Guard Baneblade can take a Monolith down.
Apocalypse demonstrates what happens when both sides come to this conclusion at the same time. Piles of rusted metal coated with guns and chainsaws, black hole grenades, tanks large enough to qualify as small cities, and Tyranid horrors that look like bad-tempered hills with spiky legs, happen.
Apocalypse just takes it Up to Eleven (and then 40,000). A Strength value of 10 is considered the highest you can go in the normal game. Apocalypse one-ups this with the D rating, which automatically wounds regardless of any modifiers and inflicts Instant Death, as well as automatically penetrating any tank it hits. On top of that, there's also the Void Missile, which includes the D-Strength value as well as having the ability to just remove any models it hits (with only Invulnerable Saves being able to save you) regardless of any rules (Vehicles and Eternal Warriors usually don't suffer as bad if they're hit by a D-strength weapon, but the Void Missile outright negate their existence). On top of that, all of the templates got doubled in size (and many of these possess the D-strength).
Emperor-Class Titans. Their primary guns are overkill against anything smaller than another Titan and nothing in the game except another Emperor is worth shooting all of the thing's guns at. If you made a model to scale with a Space Marine the thing would be the size of a 10-year-old player. The aforementioned primary weapons are each similar in size to a Baneblade.
Planetstrike: possibly the only 40K expansion suited for less than 3000 points in which dropping fragments of a starship on your enemy, blasting a Frickin' Laser Beam from orbit into the heart of their base, and pummelling them with a meteor shower, are all standard tactics. (The ship fragments, in the inaugural Planetstrike battle report, were described as going through a Guard Valkyrie flyer "like a sledgehammer through a pane of glass". Ouch.)
Two races in particular employ this level of thoroughness as one of their hats: The Tyranids will devour animals, vegetables, and minerals, as well as the geothermal energy within a planet, leaving it a lifeless husk, while the Necrons will systematically destroy all life on a planet right down to the last bacterium, to destroy anything that could even theoretically be corrupted by the Warp.
The current Blood Angels are like this with dreadnoughts: With a certain weapon equipped, a Dreadnought is fully capable of an infinite number of attacks, so much so that the rules actually had to list "this stops when you've either had a bad roll or everyone else is dead". They also have a Spellcasting Dreadnought; a Cyborg/mecha/battlesuit that can cast spells.
The Space Marine Apocalypse-only tank Terminus Ultra Land Raider. It has 8 Anti-tank cannons. The Imperial Armor Helios Land Raider might be a better example, fully upgraded it mounts 4 of the same anti-tank cannons, a long-range artillery rocket launcher, an even longer ranged cruise missile, a tank-killing heat ray, and a recoilless grenade launcher machine gun for point-defense.
Chaos runs with this with their champions: they will "bless" you to the point of overkill, and if you can't take it you get turned into a mewling spawn of chaos. If you can take it, you become a Daemon Prince, an ungodly powerful creature of death. Tzeentch is even more in love with this trope, since he is the lord of change after all.
The Tau Manta gunship, the largest single model produced by Games Workshop/Forge World (the Emperor Titan is way too big), mounts a pair of immense railguns that can punch through Titan shields, six massive anti-infantry ion cannons, a missile launcher, sixteen plasma gatling guns, and ten remote-guided cruise missiles. Its transport bays are designed to carry fifty infantry, four main battle tanks, and eight guys in flying battlesuits; but a strict interpretation of the rules allows the gunship to carry a hundred and seventy nine infantrymen.
Even the most basic ork likes to carry an ax big enough to cut a man in half with little effort. When guns enter the equation, walls of lead and steel will regularly cross the battlefield. And it only goes up from there, until you get gargants, Humongous Mecha that are basically made of gun, and carry an appropriately huge ax. The only reason they have little mention in this page is because it'd be too long; everything an ork does is overkill in at least one way.
Eldar Blackstone Fortress. They were designed as anti God weapon. The fact that a dozen of them failed to wound a single C'tan doesn't make it any less impressive.
A fluff piece in the Imperial Guard Codex talks about how the Death Korps of Krieg stopped a rebellion by bombarding the rebellious Hive City in question nonstop for ten years... Five years after the leaders of the hive transmitted their unconditional surrender, and two years after all life sign readings within the hive had ceased.
Even the Imperium's methods of exterminating life on a planet (in itself already a hilarious example of overkill) can be this. Their most basic exterminatus weapon is a simple solid slug that just obliterates everything on the surface through the sheer shockwave. The one after that is a virus bomb, where the initial virus instantly goo-ifies whatever biomatter is on the surface, then the second stage of the attack ignites the gasses released by the goo, effectively immolating the entire planet into charcoal. The third method involves shooting a melta torpedo, a fusion bomb with the heat of a thousand suns, into the core of the planet, utterly obliterating it into galactic pebbles. And if the Inquisitor is feeling particularly thorough, they can do all three just to make sure absolutely nothing is left (given the setting, there will often be something left even after all that).
Warhammer without the 40K is pretty big on the overkill factor, as well. The Orcs aren't quite as obsessed with dakka as the Orks, but only because they've been too busy beating the crap out of everything else to invent it themselves.
Noted in the DM's guide to Legend of the Five Rings is Ryoko Owari. The governess's guiding philosophy is that minor nuisances are to be ignored, but any major problem must be dealt with as much force as she can muster: "Don't resort to violence quickly, resort to violence thoroughly."
GURPS: Ultratech has a few wonderful toys. There's the ghost particle beam that makes a massive antimatter explosion inside of the target. Reality disintegrators erase people from existence. The best example might be handguns that fire nuclear weapons, which are so much overkill that they have literally no practical use.
For vehicles, there is a 100mm gatling that shoots strategic nukes - average damage for that one is 210 billion per second (the human body is vaporized around 100 damage) On the defensive side, stasis fields can take infinite amounts of damage. There are weapons that can send someone to some point in space far enough away that you can just forget about them. Other supplements have their moments too (a spell to make a volcano, a template of a very minor Eldritch Abomination). Also, in GURPS, everything can be scaled up if the GM lets you.
The Azrael from GURPS: It attacks the enemy with a bunch of 700 megaton missiles and then smashes into the planet with enough force to wipe out the dinosaurs all over again.
The massive Ogre cybertanks from GURPS Ogre, whose main guns fire SATNUC (SATuration NUclear Cluster) rounds — each round splits into multiple submunitions which detonate over the target into semi-aimed shaped nuclear plasma blasts. Ogres also carry hypersmart missiles (almost smart enough to have opinions) and use APFSDSDU (Armor-Piercing Fin-Stabilized Discarding Sabot Depleted Uranium) rounds for point defense and anti-personnel fire.
Any one of a hundred weapons or skills in Exalted would qualify, but of particular note is a supernatural martial art move called Illustrative Overkill Technique, which allows an Exalt to kill an opponent in such an excessive fashion that everyone else has to run away or start retching.
The Five-Metal Shrike. Ground zero damage for the Godspear: infinity.
Any of the three remaining Titan-class aerial citadels. Main weapon: the Godspear, only more so. Let's put this in perspective: the Shrike can fire the Godspear to deal infinite damage to an area in a 25 yard radius, once per day, with a secondary blast of 50 damage out for 500m. A Titan citadel can do the same, once per hour (during daylight), from an orichalcum lens a mile in diameter, to level an area TEN MILES ACROSS. And the hull is covered with smaller weapons to see off threats that don't mandate the reduction of an entire city to a smouldering crater. And that, ladies and gentlemen, is how you destroy Gem.
While this is all over the place, Malfeas (and any Infernal using his Charms) has a particularly impressive line in overkill, starting with being able to add lots of dice to ostentatiously brutal moves and moving up to things like turning into a walking Fantastic Nuke surrounded by a zone of horrific radioactive death or condemning enemies to ten thousand years of suffering.
Scion's Sun Purview has a Boon called "Fusion" which has a base of 7 Aggravated damage, plus the successes of a Strength+ Science roll. Seeing as 10s count twice, and you could have a pool of 15 dice, and then additional Epic Strength bonuses... and possible Arete (if you're Dodek). It's basically Hiroshima and Nagasaki on your target.
Mekton has nuclear bullets. Nuclear bullets? It also has, among other things, MIRV remotes packing enough missiles to make Honor Harrington jealous (oh, and these can be nuclear too); the Core Cannon, which is a hundred times too large to fit on the average mecha and could vaporise one seven times over in one shot; energy beams that can hit every part of a target a potentially infinite number of times; power reservoirs that can absorb your enemy's blasts, then project them back in a bolt of horrible doom that outshines the sun; and Excessive Scale, which is more or less reserved for building Death Stars, planet-sized battle fortresses, and the Tengen Toppa Gurren Lagann. The sourcebook that introduced most of these systems introduced several harder-to-damage kinds of armour plating to make up for the increased quantities of damage.
Magic: The Gathering: every colour except blue (which goes for subtlety). Black and White tend to just vaporise things (Damnation and Wrath of God), while Red can do an appalling amount of damage in the right circumstances (Shivan Meteor can do enough damage to turn an Eldritch Abomination into coleslaw, and Obliterate will kill everything except enchantments that can even theoretically be destroyed). Green's overkill is in the form of huge, bad-tempered monsters, such as Omnath, Locus of Mana (whose maximum power is limited only by how much mana you can generate between summoning him and victory), and Rampaging Baloths, which can produce a swarm of monsters that eat bears for breakfast. Available to anyone are the Eldrazi, such as Emrakul and Kozilek, each of whom is 2-4 times as powerful as the average dragon. And then we have Lux Cannon, which can be used to destroy anything that can be conceivably destroyed. The flavor text describes this trope perfectly: "There are few problems that can't be solved by putting a hole in the world."
Even Blue has some forms of mass destruction; case in point, Upheaval, which completely cleans the battlefield. Sure, the cards are returned to the player's hands instead of being destroyed, but because of the "maximum hand size" rule, probably a big chunk of those cards will go to the graveyard anyway.
Even with a deck carefully constructed not to be capable of infinite damage, it is possible to deal approximately 2 -> 34 -> 221 damage on turn one. That number is so big that only notations like chained arrow notation are capable of representing it.
With M13 comes Worldfire, which flat out negates the existence of everything that's not in a player's deck at the time and makes everyone vulnerable to the cherriest of taps. On the flip side, there's also Omniscience and Enter The Infinite. With both cards, you can literally play your entire deck in a single turn. However since you require 10 whopping mana to do so, usually you would have won anyways by that time with more conventional methods.
Rifts: The "only hitpoint that matters is the last" rule is played deadly straight for supernatural creatures even in the fluff, which means that professional military and police forces use this as standard operating procedure. It even has an In-Universe name, "misting".
Paranoia has plasma generators, nuclear cone rifle rounds, nuclear hand grenades, all of which not only kill the target but reduce it to a thick yellow spray of component atoms. In 2nd Edition, where the damage table went from 1 (may be stunned) to 20 (probably vaporized), nukes had a damage rating of 30 (standard-issue armor will not even come close to saving you).
And then there's the Warbot Mark IV◊, rivaled for size only by Alpha Complex itself (and maybe a Giant Radioactive Mutant Cockroach or two).
In Car Wars, nuclear weapons were described as something along the lines of 'set even a small one off and the game is over'. The maps were poster-sized, two feet by three feet or so, and usually represented about eight city blocks or so. The smallest available nuke would utterly destroy at least six of those maps in every direction.