Ghost Features (3.5e Variant Rule)
From D&D Wiki
Ghosts usually have powers related to the way they died - a ghost that was burned on the stake may has an amount of Pyrokinesis, a ghost that froze to death may has an amount of Cryokinesis, a ghost that drowned may has an amount of Hydrokinesis, etc.
Plus, if a ghost had some supernatural powers while they were alive, these powers most likely grew stronger (telekinesis became greater telekinesis, dimension door became teleport (without mishap chance) etc.).
As rule of thumb, vengeful ghosts (ghosts that stay on earth for revenge, usually on the ones that killed them or the ones they see as responsible to their death) are more powerful than normal ghosts, and violent ghosts (ghosts of people like serial killers that were executed etc.) are even more powerful than vengeful ghosts - the power of ghost is in direct line to their emotional state: emotions like rage, anger and vengefulness will majorly empower the ghost.
Many ghosts that remain on earth for too long may become vengeful, with their memories become twisted and their perception become tainted with anger and frustration - eventually, they begin to resemble wounded animals, attacking everyone they encounter.
Banishing or Destroying Ghosts
Obviously, it is difficult to kill what is already dead. However, there are a few ways to take care of problematic ghosts. However, there are many kinds and types of ghosts, some possessing weaknesses which are not possessed by other kinds.
Traditional Spirits is a collective namesake referring to certain kinds of ghosts that follow common rules. Traditional Spirits cannot touch iron (including cold iron and other metals that behave like it) and salt. A direct contact with either abolishs their manifestation for a short while (between 1 minute to 1 round, depends how powerful the ghost is), or forces them to leave a possessed body. They also cannot walk on consecrated ground or they will be destroyed (powerful Traditional Spirits, and Traditional Spirits of Good/Neutral clerics, are immune to that effect).
Another group of ghost kinds are Relics, which are quite similar to Traditional Spirits, even possessing simialr weakesses, however are also extremely sensitive to silver (including alchemical silver, mithral and other metals that bahave like them), which is even more marmful for them than iron and salt, and can even be used to ward them off.
Most ghosts, including Traditional Spirits and Relics, can be forced to move on by burning their physical remains. However, some ghosts attach themselves to objects who were important to them when they were alive. In such cases, those objects needed to be burned too to force the ghost to move on.
The only kind of ghosts that cannot be ward off or disposed of are Living Ghosts, because in the process of becoming living ghosts they are partly resurrected, enough to make them closer to life than other ghosts, but not enough to make them truely alive. Burning their remains is ineffective as well, as their very existance attaches them to earth.
However, all ghosts have one common weakness - other ghosts. Ghosts can destroy weaker ghosts by absorbing their energy, which is both forcing them to move on and leaving them such damaged they would never could be resurrected using mortal's magic (and characters with divine ranks have only 5% per divine rank they have to succeed). If two ghosts fight, the weaker ghost will be affected similarly to what explained above. If the two are equal in power this will happen to both of them.
The only way to actually destroy ghosts for good is to use a method that destroys the soul (as ghosts are, in fact, souls with baggage).
Ghosts of Outsiders
While most Outsiders (except Native ones) cannot become ghosts after death, sometimes they are cursed into becoming ghosts by transforming them into ones while they are still alive. However, only very powerful (and mostly malevolent) spellcasters can do so, and some Necromancers sees it as a good way to acquire powerful, yet resentful, servants.
Outsider ghosts are immune to burning their remains (since there are no remains to burn - their body was transformed) and might have a certain resistance, or even complete immunity, to other ghostly weaknesses, depends how powerful they were before they were cursed. Regardless, since most outsiders are powerful compared to other creatures, outsiders that were cursed into becoming ghosts are usually more powerful than the common ghost, and can have some unique powers and abilities.