Just discovered this beauty of a thread, and like to add my own campaigns:
Mage Wars
Centuries ago, the Mages waged their wars. From their towers, constructed both using their own magic and the engineering knowledge of the Dwarves, they waged war upon eachother.
The lesser mages soon fell, as did the temples of the priests, and everyone and everything else that happened to be in the line of fire. Only once the remaining mages where out of reach did the war halt.
The world's population was decimated. Races intermingled, orc mixing with human, halfling mixing with elf. Over time, the concept of Gods was forgotten, people focussing on everyday life, with the constant threat of a Mages Tower reminding everyone that the use of Magic of any kind was a sure way to get you killed. Swift, Violently, and without concern of bystanders.
Now, something has changed. A small group discovers that there are magic users active again. Some of them among their own. The effect is local, which is made painfully obvious when they enter the realm of another mages tower and witness the destruction of an associated magic user there.
Theirs is the task to gather information from the ruins of the small mage towers, to rekindle the believe in the Gods, to fight the evil magic users that rise alongside them, and to find out what happened.
(campaign with an abundance of half-races, with the 'evil' race being Dwarves (because they helped the Mages build their towers). PC's used currently include a half-ogre barbarian, a half-stonechild Cleric, an Elan Psion, a tiefling Wizard, an elf Ranger and a Draconian Dragon Shaman. )
Time Campaign
A small group of neanthertalls go on their quest to bring down the mighty mammoth.
Then, after centuries, stoneage people rember their former lives and adventures as Neanthertalls.
Again, centuries later, memories are awakened, the same group realising they have met before. They start to wonder about the reason, and start encountering others that live beyond the single life. Where does it lead to?
(A campaign with rotating DM's, each subsequent DM starting after several 100 years with the same characters, but (obviously) in a new time period, in a semi-historical setting located in the Netherlands. We've gone from neanthertalls 'till 30 BC now, and we're not sure where it will end....)