Here's one my former DM told me he did on a group of players he had. In his game world he had a ghost sorceress who was seeking power and preferred to strike at her enemies indirectly. The party first encountered her when they, and a paladin NPC, managed to thwart her plans. In retaliation she went back to the paladins home town and possessed his wife. When the party returned she attacked and left the paladin with no option but to kill his own wife - however, she vacated the body just as the killing blow fell so his wife would see her husband kill her and thus be unwilling to return via a raise dead spell.

The DM had intended this to be a throw away character as part of a side quest, but the players decided they absolutely had to eliminate her. So they set out to find a way of dealing with her for good. To monitor their progress she arranged for them to acquire a ring which could cast augury on demand. However, since this was 3.0, the identify spell only told them the lowest level of enchantment on the ring. It also had a second enchantment on it, I forget the name, but it meant that any scrying attempt on them would always succeed no matter what spell or item they might use to hide themselves.

As a result, she was always one step ahead of them. If they showed up at dungeon X to obtain McGuffin Y she'd teleport in ahead of them and take it.

Since she could possess people she would occasionally possess party members. Generally the DM would handle this by handing out notes to all PCs with one saying they had been possessed and the rest saying they haven't been possessed. However, he would also sometimes hand out notes to all players with all notes saying that they haven't been possessed - and watch the players go paranoid trying to work out exactly who was now possessed.

The campaign ended with two PCs still alive and in possession of an item that would trap souls. The idea was to wait until someone was possessed, kill them and trap both their soul and that of the ghost. The two players were each convinced that the other was currently possessed and battled it out until one of them was dead. They were wrong, neither was possessed but as soon as there was only one PC standing she possessed them.