Roleplaying campaigns consist of a long series of adventures set in the same world, often with the same characters. Some of my campaign own campaign settings have been run over the course of many years, with different groups of players.
The longest running have all been fantasy campaigns, mostly due to those being run back when everyone had more free time. The first was The Land of Kythe, which grew out of some maps I sketched between GCSE exams, and was ran during school and university, initially using AD&D 2nd edition then my own system Myths. Very little information has survived on this campaign due to the file formats no longer being supported.
Fantasy
Habisfern
The most completely detailed setting available here, Habisfern is a fantasy setting inspired by the Anglo Saxon Chronicles and designed for use with the Yags game system. It is a low fantasy world based on dark ages Europe.
Twilight Saga
This was an Ars Magica campaign started as a hybrid of 3rd and 4th edition, and set in the Stonehenge Tribunal.
Science Fiction
Traveller
Traveller is one of the oldest SF RPG campaign settings,
dating back to the 1970s. It has gone through several face lifts
since that time, but the setting preferred here is based on
GURPS Traveller, which is itself based on Classic Traveller - i.e.,
the original setting before the upheavals added in to make it
'more interesting'.
Mortals
By the 31st century, humanity has come to the conclusion that we
are alone in the galaxy. Thousands of worlds have been colonised,
but Earth is a distant memory, lost in the confusion of wars that
nearly wiped out humans, and which were started by a post-singularity
civilisation that we spawned, but which we no longer consider human.
Much of Known Space is ruled by the immortal Great Houses,
themselves descendants of the richest and most powerful dynasties
from before the wars.
The One Time
A hard science fiction setting with the focus on exploring the
technology and background of a campaign where humanity has gone
to the stars, though as beggars, refugees and slaves - the last
survivors of a First Contact that did not end well for us. The
One Time is a great diverse culture of alien civilisations
linked by space-time wormholes spanning billions of years.