Imperial Ordnance Pieces
"Infantry win firefights. Tanks win battles. Artillery win wars."
- – Old saying among Imperial Guard artillery officers, and even older saying among real-world militaries.
Massed infantry charges are all well and good, but sometimes flashlights and t-shirts won't cut it, no matter how many you bring. When the Imperium of Man really needs to make an impression, they call in the big guns.
Those guns don't move themselves, so they get mounted in stripped-down tractor chassis (usually a Chimera, though some artillery pieces use a Rhino or Leman Russ chassis), which moves them at a crawl that just matches the glacial pace of most Imperial campaigns. It also leaves them rather vulnerable to attack in direct combat, but they shouldn't (normally) be anywhere near the fight -- their main purpose is to make a moonscape out of the enemy's position.
The Imperial Guard is the most prolific user of artillery, though they are not unknown among the other Imperial armies.
Basilisk Artillery Gun
The Basilisk Artillery Gun is the iconic Imperial Guard artillery piece. It has the firepower to destroy almost any infantry, vehicle, or fortification, and it is simple to make and use, so it gets made in large quantities. Its long, relatively thin Earthshaker cannon is a common sight on major Imperial Guard deployments (and most IG armies on the tabletop, for that matter), and in many ways it is emblematic of the "big guns" aspect of the Guard.
The Colossus Bombard is the big daddy of the field artillery. Firing shells the size of a man, anything struck by its colossal Siege Mortar will disappear, cover or not. It is in fact so big it requires a special loading arm attached to the chassis of the transport to load shells into the weapon. Because of the size of the gun it cannot be mounted onto a Chimera chassis; it uses the chassis of the Leman Russ Battle Tank instead.
Griffon Heavy Mortar Carrier
The Griffon Heavy Mortar Carrier is the lightest ordnance piece the Imperial Guard fields, and it does exactly what its name suggests: it carries a heavy mortar capable of destroying entire squads of medium infantry in a single shell. Because of the modest size of the weapon it fields, the Griffon can afford an on-board targeting system so that it relies less on allied intelligence via vox channels. Combined with the light payload the weapon fires, this makes the Griffon's bombardments more accurate than that of any other ordnance piece the Guard can field.
Medusa Siege Gun
The Medusa Siege Gun is less about lobbing large shells into the ranks of the enemy and more about directly putting holes in enemy fortifications and tanks. The Medusa does this well; although it's not as long-ranged as other pieces of ordnance, its large shells can turn anything from bastions to groups of enemy infantry to dust in a single shot.
The Vindicator is the only self-propelled gun fielded by the Space Marines. Marines very rarely move as slowly as an Imperial Guard front line, so they usually don't use artillery at all, but occasionally they need support to crack hard targets, like fortress walls. The Vindicator fills that role: its Demolisher cannon is powerful enough to flatten any bunker or tank, while light enough that the Rhino chassis can carry it at a reasonable speed. The downside is that, because it is so small and fires such powerful explosives, there is very little room in the shell for propellant, so it has a very short range -- shorter than any other cannon (and shorter than many man-portable weapons, even).
The Medusa, AWESOME pattern.