It has been home to many nefarious villains including General Zod and his conspirators. The Phantom Zone has been used or discovered by other cultures and been referred to as the Still Zone (White Martians), Ghost Zone (Prometheus), Buffer Zone (Bgztlians), Stasis Zone (Loophole) and Honeycomb (Queen Bee). The villain Prometheus used it as a base of operations and the White Martians used it to hide their invasion fleet. They were later imprisoned there by the Martian Manhunter using Superman's Phantom Zone Projector.
The Phantom Zone has not just been used as a prison or a place to hide. Superboy used it to save the life of the hero Mon-El when he was dying from lead poisoning, since time does not pass in the Phantom Zone, and there he would not succumb to his illness.
In the Pre-Crisis DC Universe, while in the Zone one does not require sustenance and becomes telepathic; they are able to observe the normal universe, but are unable to interact with it.
Superman once possessed an arrow that would trap whoever it struck in the Phantom Zone, Roy Harper then stole it but never used it rather passing it down to his successor Mia Dearden who used it to entrap Superboy-Prime. He proved to be too powerful and managed to break free of the Phantom Zone.
Inmates or Other Inhabitants
Phantoms: Hate that others use their dimension as a prison where their worst criminals are imprisoned and so they seek revenge on those responsible.
General Zod: Perhaps the most famed criminal in the Phantom Zone he is the one most commonly portrayed in television and movies. He is seen as the one who destroyed Krypton and for that reason was sentenced to dwelling in the Phantom Zone for eternity. While there he and Ursa had a child, Lor-Zod who eventually managed to be free of the Phantom Zone.
Ursa: Originally banished to the Phantom Zone with Zod and Non. Ursa is now General Zod's lieutenant on New Krypton. She is the mother of Lor-Zod (Chris Kent)
Jax-Ur: The self-proclaimed "worst criminal in the Phantom Zone" he is the first to actually be exiled to it. The Kryptonians did this because of the fact that he performed illegal experiments that resulted in the destruction of one of Krypton's moons.
Mon-El: Mon-El, a traveling companion of Superboy once contracted lead poisoning and though he was temporarily granted a cure it eventually began to wear off. So Superboy placed him in the Stasis Zone (only later realizing what it was) where Mon-El remained so that he could continue living.
Lara Lor-Van: It has been stated that Jor-El have broken her free multiple times from the zone in the new 52. Presumably for murder.
In Smallville, the Phantom Zone is a prison that can only be accessed through crystals. Jor-El imprisoned the worst prisoners from all the 28 known galaxies there and when he died with Krypton he sent his apprentice, Raya there in case anything should happen, to keep the prisoners in check.
In the Phantom Zone, prisoners who spend enough time in there become Phantoms, beings that can possess others and are only harmed by Jor-El's crystal. Also it seems Kryptonians (and perhaps other species) become powerless in the Phantom Zone.
Some ways to create portals to the Phantom Zone are through bracelets possessed by General Zod and his disciples, the Fortress of Solitude, and with blood of the House of El.
When Clark opened the gateway he accidentally released several criminals from the Phantom Zone onto earth where they murdered many and Clark made it his personal mission to send all criminals back to the Phantom Zone (with the crystal) or kill them.
Beings who were known to at least at some time be imprisoned in there were:
Kal-El: Imprisoned there by General Zod when he refused to become his servant, accidentally released many other criminals to wreak havoc on Earth.
Raya: The "Gatekeeper" of the Phantom Zone, she was the apprentice of Jor-El and imprisoned there to save her from Krypton's destruction and to watch the criminals.
General Zod: Destroyer of Krypton, he was trapped there but freed by the joint efforts of his Disciples, Brainiac and Kal-El (who unleashed him unintentionally). He was eventually sent back to the Phantom Zone after being "torn out" of his vessel, Lex Luthor, when the crystal was put in the palm of his hand by Kal-El.
Nam-Ek: Kryptonian. The male Disciple of Zod. He was sent into the Phantom Zone by Kal-El where he was eventually killed by Raya.
Aethyr: Kryptonian. The female Disciple of Zod. She was sent into the Phantom Zone by Kal-El but what happened since is unknown.
Gloria: Her powers are similar to that of Poison Ivy and she was imprisoned for killing her whole urban planet with the plant life that she manipulates. In Smallville she stole power from males by using her plants, plants that could harm Kal-El. She was electrocuted by Kal-El and killed.
Baern: He became a Phantom in the Phantom Zone and on Earth possessed a boy from the East Coast. He could sense and absorb energy, usually radioactive, and expel it in powerful blasts. He fought both Raya and Kal-El eventually gaining power from a limitless power source, Brainiac. He then killed Raya but was sent by to the Phantom Zone by Kal-El.
Aldar: He was found by Kal-El in Seattle as he had killed thirty or so people when he ripped out the bones and feasted on their marrow. He appears to be far stronger than Kal-El and was about to kill Kal-El when the Martian Manhunter killed him.
"Dr. Hudson": A Phantom. He attempted to possess Kal-El, but his Kryptonian nature made it a battle for the Phantom to possess Kal-El. "Dr. Hudson" had to convince Kal-El, his life was a lie, so that he would give up on it, but the Martian Manhunter helped Clark expel the Phantom and then sent him back to the Phantom Zone.
Titan: He is a warrior who kills for sport. He was put in an illegal cage match, life or death, gladiatorial combat arena where he killed "meteor-freaks" from Belle-Reve. Kal-El wished to fight him and when there fight came, Titan seemed to be far stronger than Kal-El but eventually Kal-El overpowered him after a huge battle, and for the first time he wanted to kill and Titan was impaled on the own spike that he kept on his wrist to impale his victims.
Bizarro: Phantom. He was created by Kryptonians but he would not be complete until he had bonded with a Kryptonian. On Earth he killed many civilians as they were inadequate as hosts for him. He nearly killed the Martian Manhunter and then proceeded to pass through Kal-El, becoming just like him, "just a little more bizarre". The two fought and Bizarro was impaled but survived to fight Clark again. With the sage advice of Martian Manhunter, Kal-El learnt that though Kryptonite makes Bizarro more powerful the sun makes him weaker. So Kal-El and the Martian Manhunter worked together to send Bizarro into the sun, where it is presumed he died.
In the book "The Last Days of Krypton" Jor-El uses a series of crystals during a solar flare of the red sun Rao. He rips a hole in the space time continuum and after doing a test he gets sucked in. Eventually Lara comes in and finds Jor-El and helps her reverse the polarity of it and it shoots him out.