Covenant vessels, Halo - Plasma weapons on Covenant ships can melt a planet's surface. The process is called "glassing" because the re-cooled surface becomes glass.
Genesis device, Star Trek II: The Wrath of Khan - Creates life when deployed on a barren world, but destroys existing life in favor of its own matrix (and is thus a weapon) when deployed on an already inhabited world.
Priors of the
Ori, Stargate SG-1 - Can eventually turn a planet into a
black hole; can also infect a planet's population with a highly contagious
plague.
Stargate, Stargate SG-1 - While the Stargate is benign by itself, it has been used several times in conjunction with other devices ("
Redemption") or natural phenomena ("
Exodus") as a weapon.
The
Inhibitor machines from
Alastair Reynolds' Inhibitor series of novels, were capable of consuming worlds over time to convert to copies of themselves, or to create weapons capable of utilising stars to destroy planets e.g. venting stellar core material in a collimated beam to burn away planetary crusts. In the same series, the "Greenfly" machines, developed by humans as terraformers, instead go rogue and start eating planets by reducing them to their atoms and rebuilding them into more such machines, as well as numerous domes filled with vegetation.
The Dahak-class battle station (
David Weber's Heirs of Empire trilogy)
Stephen Baxter's
Moonseed: a virus-like microscopic object (or substance made from it) that transforms substances into more copies of itself - and thus consumes Venus and then the Earth by doing so. (Baxter has also employed
geomagnetic storms (see Sunstorm) and larger
universal constructors (see Evolution) as planet killers.)
Device Ultimate in The Xenocide Mission
At least five methods in
E. E. "Doc" Smith's
Lensman saga: "super-atomic bombs"; a "nutcracker", consisting of crushing a planet between two others; a "negasphere," an
antimatter planet; "Nth space planets" from other dimensions can be used to ram planets or even create supernovas - there was even the worrying possibility that these could cause the
Big Crunch in zero time; and a "sunbeam", a way of concentrating most of a sun's energy output into a narrow beam -- this one a defensive-only weapon against nutcrackers and negaspheres.
In E. E. Smith's
Skylark of Space series various planet-killers are used or discussed. Throwing planets and moons out of orbit, incredibly high-yield atomic or copper bombs, near-instantaneous dematerialisation of physical objects and the teleporting of close to fifty billion stars in order to wipe out a Galaxy-wide alien civilisation are all used.
In L. Ron Hubbard's
Battlefield Earth a device is created which, when activated, causes all matter it touches to break down into its constituent molecules. This device was used on a moon, which was consumed faster than ships based on that moon could launch.
a bomb made of 9th-dimensional matter in Supernova
In
Orson Scott Card's
Ender's Game, the MD (Molecular Disruption) Device, or "Doctor Device", generates a field inside which it is impossible for atoms to coexist in a molecule. The field propagates in a chain reaction, so it basically destroys all matter until it reaches pure space. This was intended for ship to ship combat, but was eventually used to destroy an entire planet.
In HALO: First Strike, a human weapon known as the NOVA bomb is mentioned. It is described as being made up of multiple thermal nuclear warheads with a central core of material that boosts its yield to a sufficient amount to destroy a planet and anything in the vicinity of that planet. In HALO: Ghosts of Onyx, the weapon is detonated on board a covenant ship orbiting a covenant world. The planet suffered damage sufficient to render it uninhabitable and the explosion also destroyed a covenant fleet.