Combi-weapon

Combi-weapons are specialized weapons used in Warhammer 40,000, mainly the Imperium of Man, Chaos Space Marines, and the Orks. A combi-weapon is essentially a bolter with another weapon integrated into it. Among the Space Marines they're commonly used by Terminators. One can only utilize both weapons at the same time in single shot mode. They come in several varieties.

Combi-bolter

Also known as a Twin-linked Bolter, the combi-bolter was basically a pair of boltguns welded together to create a more dangerous weapon. It has been replaced by the Storm Bolter among the Imperium, but is still used by Chaos Space Marines.

Combi-flamer

A boltgun with a flamer attached to its underside. Especially useful for close combat situations.

Combi-meltagun

A bolter with a meltagun attached to its underside. Very useful when hunting tanks.

Combi-plasma

The most prevalent type of combi-weapon among loyalists, combi-plasma guns are extremely useful. A popular tactic is to fire a plasma blast first, allowing the plasma to burn through whatever armor is present, then fire a bolt, which will be able to pierce the exposed flesh and explode inside the target.

Combi-needler

A weapon used by the Ordo Malleus, a combi-needler combines a bolter with a Needler.

Condemner boltgun

Used by Ordo Hereticus and Sisters of Battle, this is a bolter combined with one-use crossbow. Yeah, it sounds pretty pathetic. The cool point is that the crossbow bolt is filled with a mix of holy promethium and some anti-psychic shit produced by the Golden Throne. So it's not only burn the witch's body, but also burn down his very soul (which is crunch wise auto-peril to any psyker target).

Executioner pistol

Used by Eversor assassins, this is (in DH) a combi-needle pistol, or (in WH40k) a pistol that fires poisonous needle shaped bolt rounds