Virtual season

A virtual season is a fan-driven continuation of a television program. While they are typically written for cancelled pograms, they sometimes run concurrently with programs that are still airing original episodes. Virtual seasons can be written by a single author, by several collaborating authors in a fandom or even reproduced episodically in a single video or series of videos.

One of the First Virtual seasons was for the TV series Forever Knight, which was cancelled in 1996 and followed by a virtual fourth season ('V4S').
Lois & Clark: The New Adventures of Superman, which was cancelled in 1997, spawned several virtual seasons, continuing the trend. The Unaired Fifth Season ('TUFS') and Season 5 ('S5') each "aired" on Sunday nights during the 1997/98 television season, while 'Season 6' followed during the 1998/99 season.
Millennium was cancelled in 1999, six months short of its millennial climax, so a Virtual Season 4 completed the storyline for fans. In 2005 Virtual Season 5 aired, produced by a different creative team, but using the Virtual Season 4 as canon. A Virtual Season 6, by another creative team, is currently in production.
JAG spawned a Virtual Season which was designed to give the fans something to do and read during the summer hiatus. It ran from 2000-2005.
Voyager Virtual Season Project ran for 2 years, starting in 2001 and ending in 2003, extending Star Trek Voyager's story an extra two years and into yet another "Lost in Space" adventure.
Still Flying was a Firefly continuation which began airing in 2005, continuing the story from where the first season left off.
Some fans go so far as to produce fan films like Star Trek, New Voyages.
There are also websites that collate many different Virtual Seasons into one place, a "Virtual Network", such as MZP-TV, Virtual Mutant, Werewolf Studios (French), The VPN, The Entertainment Network, and GypsyVerse Productions.
The television series Supernatural has spawned five thriving Virtual Series: The Supernatural Virtual Seasons (2006–present), which is currently in its fourth season; Multus Tempestas (2006–present), which is also currently in its fourth season; Sam the Series (2008–present), which is preparing to wrap up its first season; Diversus Semita (2009–present), which currently resides in the hiatus between its first and second seasons; and Carry On (Jan. 18, 2010–present).