Melta
Melta weapons are a type of anti-armor weapon used widely by the forces of the Imperium and Chaos, the Tau and Eldar also employ their own variants. Melta weapons work by super-agitating the air around it's barrel at a sub-atomic level that it heats up to the point where it's hot enough to melt anything (From power-armor to super-heavy tank armor) and shot out. Think of microwave oven crossed with flamethrower crossed with a cutting torch.
The problem with melta-based weapons is that due to the super-high energy consumption and heat generation, it's only usable at short range. In comparison to the equally useful Plasma-based guns, Meltas are a lot stronger but have a much shorter range, but then again Meltas don't explode in your face on a bad roll. Though suited towards tank hunting, the Melta-based guns are good weapons for all-around attack, defense and infiltration, though it falls short in terms of range. Infiltration with this can take vehicles out of the fight before they even have a chance to fire their weapon...is that how the baneblades were lost?
Imperial Melta Weapons
Inferno Pistol
The Inferno Pistol is a rare pistol-sized melta weapon. Only the Ordo Hereticus and Sisters of Battle get to use them. Their range is so short that you would have more opportunities to perform a root canal on your enemy in the middle of battle than use them. Still, those big-hatted pastors love nothing more than the look on your face when they one-shot your Leman Russ. Blood Angels and their successors have a variant called the Infernus Pistol. Sanguinius used one in combination with his Power Sword for close combat, and Dante currently wields an Artificer Infernus Pistol known as the Perdition Pistol.
Meltagun
The meltagun is the standard armor-breaching "special weapon" available to Imperial armies -- Tactical Squads of Space Marines can take one, and Special Weapon Squads of Imperial Guardsmen can take three.
Multi-Melta
The multi-melta is essentially two meltaguns strapped together and supercharged, and in game terms only doubles the (already sucky) range and makes it a heavy weapon. This thing is still beastly and tears apart single targets, vehicles or otherwise, but are so huge that only Space Marines and the Sisters of Battle can lift and move them by hand. It should be noted that Sisters of Battle love Melta weapons and, if given the chance, would only ever carry these things and Flamers.
Melta Bomb
For millennia, the Adeptus Mechanicus hypothesized that there was a melta weapon with even less range than the meltagun, and sure enough, one day they found the melta bomb STC printout. Melta bombs are land-mine-sized fusion charges used to destroy enemy vehicles and fortifications; because they have to be placed next to the target to be effective, they are mostly given to assault units who will get up close and personal with the enemy on a regular basis.
Melta Cannon
The Imperial Guard felt left out, and decided to strap two Multi-Meltas together and show up the Sisters by making the Melta Cannon, an EKS BAWKS HUEG Melta weapon strapped to the Devil Dog variant of the Hellhound tank. This thing dumps small blasts worth of Multi-Melta statline upon anything it deems unlucky today, and is one of the best ways of dealing with Terminators. Titans get an even bigger Melta Cannon with absolutely rapetastic statline - it's S10 AP1 10" (yes, TEN inches) blast that always roll extra D6 on partials and 2D6 on direct hits, and due to being primary weapon it also allow you to reroll primary to-pen dice. The only two downsides is medicore (for a titan) range and no ignore cover, which is silly, because fluff-wise this thing would melt through any wall and bunker like it was made of butter, not to mention some silly ruins and trees.
Magna-Melta
The magna-melta is the Caestus Assault Ram's main ranged weapon. It is primarily intended for cutting through spaceship hulls to deposit its boarding crew inside, though it turns out to vaporize soldiers just fine on the ground.
Melta-Cutter
The Death Korps of Krieg's Hades Breaching Drill uses a melta-drill to dig through the ground. This melta-cutter is also extremely effective at liquifying units above ground.
Eldar Melta Weapons
Eldar employ the use of Fusion Guns, Fusion Pistols and Fire Pikes. These things are like more snipery versions of Meltaguns, Inferno Pistols and Multi-Meltas respectively. Unlike the Sisters of Battle, these usually come stardard and in full-squad compliment. These things are also a great way for Fire Dragons to compensate.
Dark Eldar Melta Weapons
Dark Eldar have the Heat Lance, a weapon that takes the best of the Lance and Melta weapons to obliterate any vehicle that gets too close.
Ork Melta Weapons
Ork Weirdboyz who have cheating dice rolls can throw a melty lightning blast capable of vaporizing Land Raiders.
Tau Melta Weapons
Fusion Blaster
The primary Tau melta weapon is the fusion blaster, the equivalent of the Imperial meltagun, only with higher range (think middle point in between of regular meltagun and multimelta). It's always mounted on battlesuits or vehicles.
Fusion Cascade
The close-quarters-focused XV-9 Hazard Battlesuit can also take a fusion cascade, which is capable of a higher rate of fire (it's Assault D3 instead of 1), at the cost of reduced strength (S6 instead of 8), though it is still as capable of penetrating armor as a regular Melta.
Fusion Cannon
Forge World produces a variant turret for the Hammerhead Gunship that mounts a twin-linked fusion cannon, which is the equivalent of the Devil Dog's Melta Cannon (i.e. a melta pie-plate) but twin-linked.