Chainsword
The Chainsword is a fuckawesome melee weapon, heavily used by Imperial forces, although the Eldar, Orks, and Chaos Space Marines also field it.
As its name mentions, the chainsword is a twin-bladed weapon with a rotating chainsaw blade in-between, that's powered by a motor mounted on the hilt and activated with a trigger on the grip.
Although the chainsword is issued to virtually all Imperium forces, it is most famous with the Spehss mahreens who widely use the chainsword as the primary weapon of their assault troops and veteran soldiers.
Variants
The Chainsword also has several variants that has been by both the Imperial of man and Xenos. Here are some examples:
Eviscerator
An oversized chainsword used by the Ecclesiarchy's priests or by the Sister Repentia of the Sisters of Battle. The Eviscerator is twice the size of a regular chainsword and so heavy that a normal soldier will need to carry it using both hands. The Eviscerator might also incorporate an exterminator, a portable flame unit that will only help scare a normal man shitless upon seeing a man holding this thing. The Eviscerator is also extra effective against armored units.
Scorpion Chainsword
An Eldar variant of the Chainsword fielded by the Striking scorpions. It's like the Imperial Chainsword but incorporates advanced equipment that increases the wielder's strength and a sleeker, less bulky design. There is also a custom variant for the Striking scorpion Exarch named the Biting blade which is simply larger than the regular one.
Chainaxe
The custom weapon of Khornate champions. It's basically a huge axe with a powerful chainsaw as a blade. The sheer weight of the axe and the fact that most Khorne Chaos Marine champions are super strong and borderline psycopaths: these are capable of ripping through most infantry armors.
Chainbrush
The weapon of choice of the Adeptus Orthodontus. It's based as a spear with the blade being a consecrated toothbrush/chainsaw blade. It purifies even the most revolting of mouths from the forces of oral diseases.
Chainfist
Like the powerfist but mounts a powerful chainsaw blade instead of a giant, power-sheathed fist. The weapon is commonly used by Terminators as a CQC weapon and to rip the armor off Space Hulks during boarding actions but are also used by regular Space Marines.