Missile Launcher
The Missile Launcher is a very simple weapon, being little more than a tube capable of firing different kinds of explosive payload, similar to the Mortar. The Missile launcher is capable of firing a variety of payloads, chief amongst which are the anti-personnel Fragmentation ("Frag") missiles, and anti-vehicle Krak missiles, though anti-air "Flakk" missiles were brought in with the advent of flyers in Warhammer 40,000 6th edition. Missiles with defoliant chemicals or chemical weaponry can and have been used, as well. The main advantages of the Missile launcher are that it is cheap to manufacture, able to take several kinds of munitions, and portable.
Imperium
The Missile Launcher is one of the most common explosive weapons fielded by the Imperium of Man, employed very commonly by Imperial Guard Heavy weapons teams, and extensively used amongst Space Marine squads of all kinds. It also sees use among the ranks of the Chaos Space Marines. Its incredible versatility in payload allows it to be a potent anti-infantry and anti-vehicle weapon when the situation calls for it. Its anti-vehicle capabilities are only outstripped by the Lascannon and vehicle-mounted weapons, and it is easy to learn and easy to use.
Back in the days of the Great Crusade, these things were the goofiest looking weapons ever, with banana-shaped top-fitted magazines (as seen in some recent Forge World weapons packs). Dark Eldar laughed at how oddly shaped and impractical they were. Seriously.
They stopped laughing after said missiles evaporated them.
Aside from the two standard Missile Launchers, the Astartes short launcher and the longer Imperial Guard version that shoots long, almost torpedo-like missiles, the Imperium also employs several other large missile weapons.
Hunter-Killer Missile
A single-use fire-and-forget krak missile packaged in a launch tube that is easily mounted to any vehicle. In-game, they are a cheap upgrade for most Imperial vehicles and walkers, as a quick way to add a bit of anti-armor punch to an armored personnel carrier or other light armor. However, being one shot only you'll never actually hit anything with it as it has the incredible ability to miss everything every time it is fired. (Unless you stick one on Knight Commander Pask's tank, BS4 with re-rollable 1's to hit and to wound, use it to stun enemy artillery or instagib a warlord of Toughness 4 or below with no Eternal Warrior)
Hellion Missile
Attached to the side of the Epic Stormblades casemate was a single shot Anti-Titan missile. Designed with an adamantium penetrator, it was there to help kick in the teeth of any titan that came around the corner, but fairly pointless against Gargants.
Manticore Missile
The primary weapon of the Manticore, the individual missiles of a Manticore can evaporate anything that is not dressed in Power Armor.
Deathstrike Missile
An explosive set to Maximum Fuck and strapped to a Deathstrike Missile Launcher, an impact of this weapon will fucking END anything hit by it. A single ICBM made of FUCK YOU out for your blood would make anyone think twice. Batteries of these helped out in the Battle for Armageddon when fired from literally the opposite side of the planet. An even deadlier variations exists as the Vortex Missile.
Ironstorm Missile Pod
Mounted on the back of an Imperial Knight, it is used against massed infantry assaults, and it is most effective against light- to medium-armoured opponents (72" S5 AP4, Heavy 1, Large Blast, Barrage).
Stormspear Rocket Pod
Another pod mounted on the Imperial Knight, it fires self-propelled projectiles tipped with armour-penetrating warheads and is used against heavily armoured infantry or moderately armoured vehicles (48" S8 AP3, Heavy 3).
Apocalypse Missile Launcher
Strapped to the back of a Reaver Battle Titan, the Apocalypse Missile Launcher does just that: spraying an area in explodey goodness capable of devouring waves of infantry and tanks alike.
Doomstrike Missile Launcher
So huge it can only be used by the Imperator Battle Titan, this is a highly accurate missile launcher with a high rate of fire. It gains its accuracy by way of leveling the entire half of the continent that the enemy is on so that his destruction is assured.
Chaos
The Chaos Space Marines Havocs use a man-portable missile launcher just like the Imperial model (presumably left over from their loyalist days). Their Titans also have the same missile launchers available, although the Cyclone, Hunter-Killer, and Manticore missiles weren't developed until after the Horus Heresy, so they aren't available to the Chaos Space Marines. Apparently they couldn't steal some after they were invented, and none of the Chapters who turned traitor later brought any with them. Its just as likely that they have never gotten their hands on any substantial manufacturing complexes for them, meaning they have to use the limited Imperial Ordanance they do get from their looting and stolen factories in apocalypse battles. In dawn of war 2, the missiles launcher used by the plague marines puts slow effects on vehicles they hit, which is fantastic for ensnaring dangerous targets like avatar of Khaine, land raider or any METAL BOXES for your damage dealing units to catch up on them.
Havoc Launcher
The Havoc Launcher is a small multiple missile pod that hooks onto Chaos vehicles (it was used on all Imperial vehicles during the Great Crusade and both sides of the Horus Heresy, though it's Chaos-only now) and can fire off a cluster of long-ranged explosive shells, perfect for dispatching clusters of small enemies - it's particularly effective against Orks, Dark Eldar, the Tyranids, Kroot, and the Imperial Guard. The weapon has the following profile:
Range: 48" | Strength: 5 | AP: 5 | Heavy 1, Blast, Twin-Linked
It can be thought of as a rough equivalent to the Imperial Hunter-Killer or Cyclone missile launchers, as it is an upgrade easily applied to any vehicle for some extra firepower, but where the Hunter-Killer missile is meant to damage or kill a single strong unit (with only one shot, mind), the Havoc Launcher is meant to handle mass infantry and attack weaker units - and unlike the Hunter-Killer missile, can be fired all damned game long. Be warned, though; Tau, 'Ard boyz, Necrons, Storm Troopers, and Space Marines can generally weather its offensive - though the Havoc Launcher will reliably cause wounding hits, its poor AP value means most forces still get their saves. It's still a decent deterrent to infantry-heavy armies (Tyrablobs, Guard blobs, and Orks), though, and not a bad investment for the points. Also, once your Rhino has disgorged its angry Berserkers and been immobilised, your opponent tends to forget that it's a threat. This can result in the paralysed transport causing the final wound to a special snowflake Space Marine character when both armies in a 800pt annihilation game are otherwise exhausted, tabling the opponent.
Eldar
Eldar Missile Launcher
Unlike the ones used by the Imperium, Eldar missile launchers consist of several pods of small missiles, using a cluster of small projectiles instead of a single large one to deal damage. Of note is that the number of openings on the launcher does not reflect the number of missiles that it is capable of holding: each firing tube is connected to several smaller chambers, in which the missiles are held and fired from. In addition, the Eldar eschew fragmentation rounds in favour of an unstable plasma core round, which has a small degree more penetrative prowess.
Reaper Launcher
The Dark Reapers aspect warriors use a variation on the Eldar Missile Launcher: the Reaper Launcher. Firing a hail of small missiles over a very large distance, they can do considerable damage to Space Marines and all but the largest of Tyranid organisms. Exarchs of the Dark Reapers can use either a regular Eldar missile launcher or the Tempest Launcher: a modified Reaper Launcher that shoots its projectiles in an arc, bombarding a large area with lethal projectiles.
Tau
The Tau also use missile launchers, albeit only on their battlesuits and vehicles -- Fire Warrior squads are instead given markerlights and are expected to designate targets for the vehicles to shoot at, rather than carry missiles themselves. This is because the hunter cadre is far more integrated, the vehicles and infantry work as one. They are generally not as powerful as their Imperial equivalents, but they can be more accurate if their targets are illuminated by markerlights.
Missile Pod
A missile pod is a small rack of miniature missiles mounted on the arm or shoulder of a Battlesuit. It isn't quite as strong or armor-piercing or long-ranged as an Imperial missile launcher, but it is an assault weapon, rather than a heavy weapon.
High-Yield Missile Pod
The High-Yield Missile Pod is large rack of High Explosive Missiles mounted on the hands of XV-88 Broadside Battlesuit in place of the normal Twin-Linked Heavy Rail Rifle, designed as a Light Vehicle killer this thing fires double the normal amount of missiles compared to its lighter cousin and spells the end of any light vehicle caught in its range.
Smart Missile System
A smart missile system's munitions have even less strength, range, and armor-piercing ability than those of a missile pod, but they include advanced built-in target tracking and guidance systems. The operator must remain steady to designate a target, but can then unleash four missiles that will Robotech around obstacles on the way to their target allowing for multiple simultaneous and devastating offensive missile strikes.
Cluster Rocket System
One of the two primary weapons of the XV12-8 Stormsurge Suit, these over grown missile pods replace the Suits arms and can vomit forth enough ordnance to threaten anything with T:7 or lower, or an AV of 11, with the ability to fire up to 4d6 attacks, or 8d6 if it stabilises at a range of 48", so long as Lady Luck, and Admiral Awesome smile on you, this thing will earn back its points, and then some solely with amount of infantry it can annihilate
Nexus Meteor Missile System
Because the Cluster Rocket Pod wasn't producing a big enough boom. This thing can only be mounted on the KX-139 Ta'Unar Supremacy Suit, and boy does it show. A swarm killer of the highest order this weapon lays down one Apocalyptic Mega Blast Template that starts out as strong as a Railgun, and ends up as weak as a Pulse Rifle, with an explosion bright enough to blind anyone left standing.
Seeker Missile
The seeker missile is the Tau Empire's primary indirect fire support weapon, equivalent to the Imperial hunter-killer missile. Prior to 6th edition, they could only be activated remotely by a markerlight, which they made up for by being BS5. The 6th edition Tau Empire codex made them able to be fired like normal weapons as well - without anyone else to give them markerlight rules, they are identical to hunter-killer missiles. With markerlights it is a slightly different story. They can be remotely fired even if the vehicle carrying them would be unable to use them (because it has moved, or does not have line-of-sight to a target, for example). The Tau Empire realized that having a vehicle carry missiles but be unable to fire them itself would be silly, and so most vehicles dedicated to carrying seeker missiles, like the Skyray Missile Gunship, mount networked markerlights that allow them to designate targets themselves. Other largish Tau vehicles can mount a smaller number of seeker missiles purely for infantry anti-armor support.
Destroyer Missile
Take a seeker missile, feed it steroids made of hate, and then give it something to destroy, there you've got a Destroyer Missile. Mounted on the KV128 Stormsurge suit in sets of 4 these missiles normally act at S:8, AP1, but can be fired at S:D by spending a Markerlight Token
Orks
Da boyz, dey like dem rokkits, 'cuz dey make some big 'splosions. When ya need ta crack open da humie metal boxes ta chop 'em to bits, get da Rokkit Launcha. Da Launcha got'z a button dat shoots rokkits at da gitz it iz pointed at. Even snots know dat. Some Boyz just carry da Rokkit Launcher like a Shoota, but da Meks dey put it on Trukks, Trakks, 'kopters... Like I wuz saying, da boyz dey like dem rokkits so da Meks put 'em errywhere!
Tankbustas, gitz ta really like ta krump' da 'umie (and panzee) 'Tanks', day also get da Tankhammer. Da Tankhammer, its just a buncha 'splosives on a stikk ta be used in melee like a choppa. Hit da humie wagon an' it goes bang.
Den dere'z dem Mekboyz dat muck about wit bigga n' blastier rokkits. Dey put Grotz in 'em ta guide da bomm. Humies call 'em "smart bomms". Any propa git knows deyze called Grot Bomms.