The standard melee weapon for Space Marine with motorized monomolecular-edged teeth capable of slicing through armour, flesh, and bone.  Fast and responsive in the hands of Assault Marine it is capable of sawing through a variety of enemies.
Chainsword
A two handed rifle that fires pulses of superheated ionized gas towards the enemy that detonate with the power of a small sun.  

While extermely effective at melting through enemy armour and vehicle, the plasma gun is also prone to building up dangerous levels of heat that must be vented to prevent the shooter from being injured.
Plasma Gun
More powerful than the Chainsword, the Power Sword sacrifices some of the additional damage of heavier melee weapons like the Power Axe with a faster attack speed.
Power Sword
An extermely powerful, but slow melee weapon that builds up an immense energy charge that is released upon impact with a target and decimates everything within its area of effect.
ThunderHammer
The standard Space Marine assault weapon that fires small missles, or 'bolts'.  Each self-propelled bolt contains an explosive charge that detonates once it has penetrated the target, blowing it apart from the inside.  Effective against a wide variety of targets.
Bolter
A handheld sidearm version of the bolter that is typically wielded alongside the chainsword.
Bolt Pistol
Most typically found amongst the Ultramarines and their sub chapters, the Stalker Pattern is a semi automatic bolter upgraded with enhanced targetting arrays and is deadly at medium to long range.
Stalker Pattern Bolter