In Warhammer 40,000, there are few things more dangerous to life in the galaxy than the Warp. Especially when it's weaponized. Some of the deadliest weapons in existence, Warp Weapons are devices capable of using the warp to deal tremendous damage to any target presented with.
Vortex Weapons are based on the same technology as the warp engines of a voidship: they basically tear a momentary 'hole' (aforementioned vortex) between realspace and the Warp. But where a spaceship is (hopefully) protected by a Gellar Field to survive the experience, anything that gets sucked through find itself at the tender (lack of) mercy of the denizens of the Warp. Those weapons are frighteningly effective, but they are used with circumspection. In theory, the vortex' existence is very brief. In practice, sometimes it somehow can sustain itself and it starts drifting around... Or growing... or things start coming through from the other side to have some fun...
Imperial sanctioned weapons come in two variations: Vortex grenades that are man-portable but whose effect is limited and the Vortex Missile which is mounted on either a Deathstrike Missile Launcher or Reaver/Warlord Battle Titans and can hatefuck much more massive holes in reality in a single strike.
An example of a non-standard Vortex Weapon is the Dominus Astra, an Emperor Class Battleship, which when strategically located and having its warp drive detonated was capable of destroying almost all of Hive Fleet Behemoth. Needless to say this is not something the Imperium is well equipped to do, as each Emperor-class is neigh-irreplaceable...
The Rift Cannon used by the Dark Angels Space Marine Chapter is also a warp weapon supposedly developed in the Dark Age of Technology. When fired, a beam of warp energy emerges that resembles a kaleidoscope of colors blossoming into a small rift in the Immaterium, opening at the beam's point of contact. The weapon is amusingly called the Stained Glass Cannon primarily due to this kaleidoscopic explosion.
Being either creatures born for the Warp or their mortal servants, the majority of weapons use by the forces of Chaos use Warp energy in one form or another. The most famous ones are the various Daemon Weapons that actually house a daemon, but many of the Dark Mechanicus more outlandish weapons are in part or entirely fuelled by the Warp directly.
Though Warp weapons are considered taboo in Eldar society for their unnerving effects, Eldar military forces always seem to find that they don't really give a good goddamn when it comes to defending the craftworlds, and as such, make use of more Warp weapons than anyone else in 40k.
Carried by Eldar Wraithguard and a certain variant of grav-tank, these guns are just like the Imperial Vortex weapon in that they open a portal and drag anything near it to the warp. They are powered by the Wraithguard's psychic energy of its spirit stone, the Wraithcannon opens a small temporary rift between real and warp space, tearing apart its target as it is torn between different dimensions.
However, unlike the D-Cannon the Wraithcannon is more tightly controlled and more accurate. The Wraithcannons are so fiendish that no mortal (only Wraithguard) could wield them. There's also the Heavy Wraithcannon, which has triple the range, and is the basic weapon of the Wraithknight.
A variant on the Wraithcannon that is dispersed instead of focused, so it can hit more enemies at once but does less damage. There is once again a heavier variant called the Heavy D-Scythe that's mounted on the Hemlock Wraithfighter.
Considered a sinister and taboo weapon, D-Scythes can rip the soul of an enemy from their body and banish it to the Warp. When fired, a D-Scythe emits no visible blast and leaves behind no damage, save the motionless corpse of those targeted. D-Scythes are normally wielded by Wraithguards in place of their Wraithcannons.
A larger version of the Wraithcannon mounted on a special artillery platform, the D-Cannon can collapse realspace in a small area and suck in all unlucky sods that stood too close to the hole. You want it.
The size of the warp hole created depends on the size of the D-Cannon, with larger weapons requiring more power and a larger chassis to mount them. The largest D-Cannons are those mounted on Eldar Cobras and Phantom Titans, and even Void Shields are useless in defending against them. These terrifying weapons are capable of swallowing multiple battle tanks in one hit, and the shockwave that results from their sudden closure can knock over other armoured vehicles lucky enough to avoid being dragged into Hell.
The AdMech tried numerous times to reverse-engineer and study the mechanics of the D-Cannon. Failures, one too many.
It also has a interesting name.
Shokk Attack Gun
The Shokk Attack Gun is one of the only Ork-portable weapons that force the wielder to stay still while firing. It opens a conduit through the Warp to the target and shoves the ammunition through it. Orks like to use Snotlings for this; because they travel through the Warp unprotected, they get scared out of their minds by the daemons, and when they come out, they usually rip and tear their way through anything in front of them. It's crazily unpredictable though. It can do anything from killing everything within D6" of the Big Mek, including the Mek himself or rolling double six and utterly destroying anything under the template.
In appearance it is a large, heavy piece of machinery with huge, spinning blades at the front and numerous whirring, grinding cogs and other gubbins sticking out from the casing. When used it makes a distinktive whine, shakes and rattles in an alarming fashion, oily smoke pours from inside as its Mek Operator holds on as tight as he can. It is very hard to aim with a Shokk Attack Gun, as even the strongest Orks find it hard to hold it still.
The Tellyport Blasta is an Ork Energy weapon sometimes integrated into Mega Armour used by Big Meks. Basically an enlarged Shokk Attack Gun that, instead of opening up a Warprift and sucking up Snotlings for ammunitions; is big enough to just use enemy vehicles and units as its ammuntion.
This is because it is based on Tellyporta technology. The weapon folds its target in a Warpspace bubble and displaces it at random. Victims will only travel a short distance before they reappear again, so the Meks use the weapon to rematerialize them high in the air, or inside a solid object. Thus a Tellyport Blasta tends to be unpredictable, but, as many other Ork weapons, still effective.
An enlarged version of this weapon is used on Ork Wazbom Blastajets.
In 8th Edition, it fires an assault range 12" D3 shots at a S8 and AP-2, this thing has an interesting way of instantly killing its victims. Any model that is wounded, but not killed by this gun, is instantly killed at the end of the shooting phase if the Ork Player rolls higher than that model's current wounds on a D6. It is quite nice albeit somewhat situational having a little bit of extra dread for those super beefy characters that were already wounded but cling to their wounds at a virtue of inv save or other shenanigans.
Kustom Shokk Rifle
The Kustom Shokk Rifle is an upsized and overcharged version of the classic Shokk Attack Gun. It is the primary weapon of the Shokkjump Dragsta and was originally conceived when the Mad Mek Gungubbinz somehow managed to fire himself out of his own Shokk Attack Gun and live, the Kustom Shokk Rifle within the Shokkjump Dragsta incorporates part of the mechanics of a Shokk Attack Gun into its engine, allowing it to teleport short distances at will. Needless to say, most of the Dragsta's drivers tend to be not completely sane.
Additionally, the Kustom Shokk Rifle is able to zap targeted units into the Warp just like the Shokk Attack Gun, although whilst the Shokk Attack Gun uses Snotlings as live ammunition, the purpose of the Kustom Shokk Rifle behaves similarly to the Eldar D-Cannons.
Crunchwise, the Kustom Shokk Rifle is a 24" Assault 2 weapon that is strength 8, AP-3 and damages on a D6. Rolling a hit roll on a 1 causes the bearer to suffer one mortal wound whilst rolling a 6 on a wound roll causes the target to suffer one mortal wound along with any other damage sustained beforehand.
The Hrud use a type of warp-laced plasma to create a potent sniper weapon called the Warp Fusil. The plasma shot dematerializes from real space for a moment, allowing it to bypass barriers and armour, before emerging into reality a moment later and making a gooey, charred mess of whatever was aimed at. Because Hrud infest everywhere and their warp fusils are excellent at assassinating things, they see use among many alien and imperial mercenaries.
The Kinebrach used to make weapons, of which at least some required interaction with the Warp or Warp entities during their creation. Some of the weapons were Cubes. The production of many, if not all of their weaponry was later considered forbidden. The Interex showed these weapons to the Imperium, despite suspecting the Imperium being tainted by Kaos (their word for Chaos). This may have been a test to see if they were actually tainted by Kaos, an attempt to impress the militaristic Imperium, or both.
Old Ones Variants
During the War in Heaven, the Old Ones discovered that the C'Tan and their Necron armies were vulnerable against psychic forces. To capitalize on this weakness, they had the brilliant idea of weaponizing the Warp, thus turning it into the twisted shit-pit it is now. Of particular note is the Shadowlight, believed to be the power source of some larger assembly. The power source alone is capable of making psykers more powerful, latent psykers become psykers, and anyone else dies a gruesome death upon unprotected exposure to it (although psykers, latent or otherwise, might also suffer said gruesome death). According to a psyker inquisitor, if Chaos got their hands on it, there would be a lot of unsanctioned psykers running around (duh), and without the Emperor's protection they would get possessed, and within 2 generations the Eye of Terror would expand so large that it would engulf the entire Imperium. It is suspected that the assembly "can alter the very fabric of reality". This suspicion proved correct, as later it opened a portal to the warp, and a daemon came out. The assembly's intended purpose is unknown, but it is suspected to be designed to somehow help the old ones in the war against the C'Tan.
Each of these weapons was a star fortress equipped with main guns that made the Death Star look like a fucking pea shooter. Drawing upon the warp and channeling staggering amounts of power, they fired beams of pure psychic energy that caused anything in their path to suffer critical existence failure. These things were anti-star weapons! During the Gothic war, Abaddon used three partially re-activated Fortresses to destroy the entire Tarantis system by blowing the local sun a new asshole, causing it to go supernova. Quite characteristically, Abaddon still manages to lose the war with such powerful weapons.
Somehow, the Void Dragon survived a combined blast from multiple of these things and all it did was make him a bit drowsy. WOW. Then the Big E kicked him in the balls. Which gives you a pretty good idea of how shit of an idea it is to piss off the emprah. Except the Emperor fought only a mere shard of the Void Dragon, not the fully-formed C'Tan, which is literally a god, as powerful in the material realm as the Chaos Gods are in the Warp.