Superweapons were created at various points in galactic history, such as the Mass Shadow Generator used during the Mandalorian Wars. World-devastating weapons often had an unknown origin, such as the ancient builders of Centerpoint Station, or came from an extra-galactic source, like the Yuuzhan Vong, although the latter's Vongforming technology more often fell into the category of bioweapons.
Another famous example was the Star Forge, built in pre-Republic times by the ancient Rakata. Arguably the most powerful weapon ever built, the Star Forge constructed an infinite armada of warships, taking energy directly from a nearby star. This had many notable advantages, including that the Star Forge could remain hidden while still conquering the galaxy, unlike most superweapons of the Galactic Empire.
One unique superweapon was the Shawken Device, with the potential to destroy the entire universe, but its method of functioning may have rendered it ultimately useless. Also, it is unknown if it was even operational.
During the era of Ulic Qel-Droma, the Sith constructed a superweapon known as the Dark Reaper. The thought bomb was designed to kill all living beings in a certain area around it. It was used in the Seventh Battle of Ruusan against Lord Hoth and the Army of Light.
Many famous superweapons were developed by the Galactic Empire. Imperial superweapons were inspired by the Tarkin Doctrine of "rule through fear of force, rather than force itself." These included the Death Stars, the World Devastators, the Sun Crusher, and the Galaxy Gun, among others. The most subtle of the Imperial super weapons was the Orbital Nightcloak.The Empire was also fond of superlaser powered weapons.
Superweapons were typically designed and manufactured by interstellar governments, as most minor factions could not afford the materials and technology required to build them. However, at certain points in Galactic history, superweapons were commissioned by powerful crime lords. Sise Fromm commissioned the Trigon One, and Bevel Lemelisk designed the Darksaber for Durga the Hutt.
It should be noted that large capital ships, such as the Interdictor-class cruisers, the Confederacy of Independent Systems' Providence-class carrier/destroyers, and the Empire's Imperial-class Star Destroyers, were capable of rendering entire planets lifeless, but not completely annihilating them. Unlike the use of superweapons, this usually required a sustained bombardment, a Base Delta Zero, a combined assault from a fleet of ships, the sabotage of planetary shields, or some combination thereof.
The Sith supernova spell, due to its effects, can be considered a superweapon, which lends credence to Darth Vader's comment: "The ability to destroy a planet is insignifigant next to the power of the Force."