The Death Star. You've all seen it, and marveled at it's destructive beauty. Originally designed to be an inter-system weapon to intimidate rebellious planets into submission, it destroyed Alderaan, princess Leia's home planet. Despite the massive PR against it, and every admiral in the imperial navy saying not to do it again, and they probably wouldn't, that bitch Leia's like "herp derp my family's dead and I don't have my favorite playground anymoar" so they launch an attack on the death star based on plans she stole and hid in R2D2. Due to a technical oversight in the plans, a small fighter was able to shoot a torpedo down a service shaft and blow it up. So the empire builds another one and it got blown up before it was finished getting built.
Centerpoint Station, sittin' pretty in the Corellia system that it created. Yeah, created. It pulled different planets, asteroids and a sun using its huge magnetic puller majiggy. It's believed to have been built by the Architects, a.k.a the Celestials, up to a million years BBY. These Celestials are also believe to have constructed The Maw black hole cluster. It's 350Km long and 100Km wide, and it has a superlaser, much like the death star except bigger, and it has an interdiction field it can project and pull ships out of hyperspace.
The only times I know of it being used are during the Yuuzahn Vong war by Thrakan Sal-Solo, Han Solo's cousin, because Jacen Solo is a dictator and Anakin Solo is a giant faggot, and Thrakan was the only one with the balls to do it, and during the Second Galactic Civil War, aka the Corellian Insurrection.
To see more superweapons, refer to the wookiepedia article superweapons.
In conclusion, FEAR THE EMPIRE!