Skylab
In 1968, the Apollo Applications Program was created to look into science missions that could be performed with the surplus Apollo hardware. Much of the planning centered on the idea of a space station, which eventually spawned the Skylab program. Skylab was launched using a two-stage Saturn V, similar to the proposed Saturn INT-21. It was the only launch not directly related to the Apollo lunar landing program. The only significant changes to the Saturn V from the Apollo configurations involved some modification to the S-II to act as the terminal stage for inserting the Skylab payload into earth orbit, and to vent excess propellant after engine cutoff so the spent stage would not rupture in orbit. The S-II remained in orbit for almost two years, and made an uncontrolled re-entry on January 11, 1975.
Originally it was planned to use a 'wet workshop' concept, with a rocket stage being launched into orbit by a Saturn 1B and its spent S-IVB outfitted in space, but this was abandoned for the 'dry workshop' concept: An S-IVB stage from a Saturn IB was converted into a space station on the ground and launched on a Saturn V. A backup, constructed from a Saturn V third stage, is now on display at the National Air and Space Museum.
Three crews lived aboard Skylab from May 25, 1973 to February 8, 1974, with Skylab remaining in orbit until July 11, 1979.
Read more about this topic: Saturn V