Force Weapons
Force weapons are psychic weapons in Warhammer 40,000, used by the Imperium of Man, Chaos Space Marines, and the Eldar. They are used almost exclusively as close combat weapons by psykers, and serve to store up and channel psychic energy. Force weapons are very useful against Daemons, as Warp energy is one of the few surefire ways to put down a Daemon.
The only army that uses force weapons as near standard issue equipment pieces are the Grey Knights. They can also have ranged "force" weapons, such as the Psilencer and its Dreadknight counterpart, the Gatling Psilencer.
In 8th edition, generally bump a weapon's damage to d3. Previously, they had a psychically-activated Instant Death. In the role playing games, they tended to kill everyone, including the user, but that was true of most psychic powers.
Imperial
Force staff
Serving as a conduit for psychic energy, force staffs are used to channel the wielder's psychic power through the staff and direct it at an opponent, usually in the form of warp fire or warp lighting. Clan Raukaan has one that acts as a thunder hammer.
Force sword
Like the force staff, a force sword gathers energy the wielder's psychic power, but instead of channeling it at an opponent, it will store the accumulated psychic power as warp fire coursing along the blade, allowing the psyker in question to attack his opponent with a flaming sword.
Force axe
Like the force sword, but as an axe.
Nemesis force weapon
Nemesis force weapons are the signature weapons of the Grey Knights Space Marine Chapter. Made of special materials containing iron and silver, the power of a Nemesis force weapon is proportional to the Grey Knight wielding it. Additionally, Nemesis force weapons are one of the few types of force weapons that are also power weapons. The most common types are Nemesis force swords and Nemesis force halberds.
Furioso Force Halberd
Furioso Force Halberds are basically Dreadnought sized force weapons used by Blood Angel Librarian Dreadnoughts.
Eldar weapons
Witch Blade
Used by Eldar Seers, particularly Farseers and Warlocks, Witchblades serve as Force Swords with a crystalline, helix shaped force crystal that is powered by the wielder's psychic power.
Singing Spear
A Singing Spear is a force spear which will store psychic energy derived from an Eldar Seer, which the Seer will then throw at an opponent. Once it has struck its opponent, the Singing Spear will return to its wielder.