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 are also on the way with the advent of flyers in Warhammer 40,000 6th edition. 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.
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. An even deadlier variations exists as the Vortex Missile.
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 it 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.
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
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.
Dark Reapers
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 (versions and tyrants hopped up on iron arm can just ignore it, trolololol.) 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 arch, 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. 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.
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.
Seeker Missile
The seeker missile is the Tau Empire's primary indirect fire support weapon, equivalent to the Imperial hunter-killer missile (except with BS 5 instead of 3). They are incapable of being fired on their own; they must be activated remotely by a markerlight, though this means that they can be 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 carry 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.
Orks
Those of us boyz who wanna crack open da humie metal boxes get da Rokkit Launcha an' da Tankhammer. Da Launcha got'z a button dat shoots rokkits at da tank. Even snots know dat. Tankbustas who like krumpin in melee can use da Tankhammer, its just a buncha 'splosives on a stikk. Hit da humie wagon an' it goes bang. Dem Mekboyz dat muck about wit bigga n' blastier rokkits put grotz in 'em ta make what da humies call "smart bomms" but any proper git knows deyze called Grot Bomms.