Chem Weapons
Chem weapons are any weapons that use chemicals (Either artificially or biologically) to break down organic or inorganic substances down to its basic, soupy elements. Basically, we are talking about acid here, the type that makes the Wicked Witch of the West scream, "I'm melting!". And no, Chem Weapons based on biological bacteria still behaves like an acid or chemical reaction. Typically the most common user of Chem Weapons are Tyranids, but they basically have their own page for that. The other common user are the Death Guard which is unsurprising.
Chem Weapons are often used as a 'cleaner' counterpart to Flamers, as most chem acids are so powerful that the resultant victim is reduced to liquefied carbon composite that can be washed away with a spray of water. Furthermore, acids have the chance of weakening certain materials, which makes it great against power armor. Of course, that also means that certain materials can be so resistant to that particular acid compound that they can just laugh it off like water. Additionally, Chem Weapons just don't have the same psychological impact as Flamers as well as the fact that they don't have the unique aspects on sucking any breathable air in an enclosed space. Then again, nobody said that they can't combine the aspects of both....
Imperium
Chem Cannon
The main armament of the Bane Wolf, the Chem Cannon smother their targets in broiling clouds of noxious gas that dissolve organic material in moments. The victims' blood boils in their own veins and their flesh sloughs from one as the mixture of toxins do their horrific work before rendering even that down into a bubbling pool of rank, festering ooze.
Unfortunately, the acidic clouds of a Chem Cannon are known to drift according to wind direction, sometimes even back towards Imperial forces. So it is in the Commissar's responsibility to maintain at least a bag of chem resistant suits for his troops to survive.
Chaos
Injector Pistol
Injector Pistols are the infamous weapon of Death Guard Biologus Putrifiers.
These weapons are only used on subjects the Putrifier wishes to study the effects of his latest strain of plague on and are often carefully selected among the enemy on the battlefield. A shot by an Injector Pistol injects a concentrated dose of Daemonic disease into the victim, causing him to erupt in explosive boils and clouds of flies. Those specimens whose deaths are especially fascinating are pierced with injector pistols once again, this time to extract whatever clotted foulness now passes for their blood, ready for later study.
Bile Spewer
Technically not a flamer since it doesn't shoot fire but is still based on it. Papa Nurgle's forces doesn't use fire-based weapons because it doesn't leave anything for decay (since the flesh is burned off). Instead, his forces use "Bile Spewers", weapons using the flamer as the base weapon but the promethium tank is instead replaced with a tank full of toxic substances that does the same job as a flamer (efficiently kill infantry anywhere, be it in cover or in the open) but leaves a chance for the target's corpse to rot and decay, just as Papa Nurgle would like it.
Usually most players would swap the Bile Spewer for something better, but for a starting weapon, the Bile Spewer is a relatively decent weapon against horde armies and light to medium infantry. Even light vehicles aren't safe from this gun.
So far, this weapon's been only featured in Dawn of War II and is a great alternative to the Imperial Guard's Bane Wolf.
Plague Sprayer
AKA, the Nurglite Water Hose. A Plague Sprayer is the primary weapon of the abominable Plague Marines of the Death Guard Traitor Legion known as Foul Blightspawn. These cruel and malignant warriors take sick pleasure in watching their victims suffer in the grip of Nurgle's plagues.
On the field of battle, these foul servants of the Plague God spread putrescence and disease with their Plague Sprayers, which unleash a stream of stinking, infectious slime that is so potent that it can melt armor and flesh alike, rotting the souls of their unfortunate victims from within. The Plague Sprayer is attached to a contraption called the malignant churn, which the Blightspawn uses to mix and prepare his foul brew, stored in an "incubatum" worn on his back.
On the tabletop, the Plague Sprayer is a 9" Assault 1d6 S2d6 AP-3 D3 Plague Weapon that autohits like a flamer, which means that with a good enough roll for its S and number of shots it can trash anything from MEQs to vehicles.
Plague Spewer
The Bile Spewer's bigger brother. Plague Spewers are a type of Plague Weapon used by followers of Nurgle such as the Death Guard. They are one of the primary weapons of the elite Blightlord Terminators, along with the Blight Launcher. They are the Nurglite equivalent to a Heavy Flamer.
In terms of crunch, the Plague Spewer is the same as the Plague Belcher but with heavy D6 S5 and AP-1. The plague equivalent of the heavy flamer. Stronger, but keep in mind that you still can't shoot it after advancing, which is pretty important given your slow movement. It also come at an ungodly price of 19pts.
Blight Launcher
Blight Launchers are a type of heavy Grenade Launcher used by Death Guard Blightlord Terminators and Plague Marines. This, along with the Plague Spewer is one of the primary range weapons of the Blightlords.
Blight Launchers are basically the Nurglite version of Plasma Cannons. As Plasma Cannons vaporize any disease upon impact, the Blight Launcher acts as a compensation for Papa Nurgle; hitting with roughly the same force as a Plasma Cannon whilst leaving a pool of toxic chemicals in which Papa Nurgle loves the best.
In terms of crunch, they are 24" Assault 2 S6 AP-2 D d3 plague weapons. A hard-hitting anti-infantry weapon that's also good against light vehicles. It can very well substitute plasma weapons if you want some more range in exchange with AP. It also has some really nice combo with Arch Contaminator.
Dark Eldar
Liquifier Gun
In simpler terms, they are the Dark Eldar's version of a Bile Spewer. More specifically however, Liquifier Guns are Dark Eldar weapons which spray a huge amount of potent acid that melt through body and armor alike. Minions of feared Haemonculi usually have built-in liquifier guns from which they can fire their own acid-like blood onto the enemies.
Just as its name implies, Liquifier Guns reduce biological and inorganic materials into a chemical soup as the chemicals within these weapons are so potent (Thanks to the advancement in biological and chemical technology of the Dark Eldar) that very few factions can rival it.
In terms of crunch, Liquifier Guns is a D6 weapon that auto-hits with AP-D3. Even the worst roll for the AP still screws with armor and that 33% chance of turning power armour into a 6+ save is fantastic. Unfortunately it only has the strength of 3.
Destructor
The Destructor is a Dark Eldar weapon used primarily by Haemonculi. It resembles a long tube, with a cavity to slip one's hand into up to the elbow. The ammunition for the weapon is carried in a small container the Haemonculus wears, fed through a series of tubes. The Destructor sprays a wide stream of an incredibly corrosive organo-acidic compound when fired. This toxin can eat through any known armour, burn flesh, and once inside the target's system can cause, "blood vessel explosion or implosion; pharyngeal contraction; extensive haemolysis; skeletal disintegration; sclerotic corrosion; intercostal spasms; hyper-reacted thermoreceptors and chemoreceptors; Eustachian damage; retinal scarring; cardiac and respiratory atrophy and aqueous humor deprivation."
This device's effectiveness in combat has been confirmed time and again, with reports of a single Haemonculus able to decimate an entire squad of Space Marines with a single well-aimed volley, every man among them dying from sheer pain, or worse, leaving them alive but immobilized - blind, hideously burned, hemorrhaging blood from every orifice, wracked in unimaginable amounts of agony, helpless and in the twisted hands of the Haemonculi.