Fanfiction is stories written by fans of a particular author or series, based on characters and situations from that series. It is always not for profit, stories written by fans and posted for free on the Internet for the enjoyment of other fans.

During the late 1990s I wrote a lot of fanfiction based on Ranma 1/2 by Rumiko Takahashi. Family, work, and other responsibilities have mostly taken me away from this, though I still get back to it occasionally.
Big Who
Who was the Doctor before he was the Doctor? (Answer: Yes.) A Doctor Who prequel based on the cartoon and book series Big Nate by Lincoln Peirce.

Episode 1: Who's Who
Episode 2: A World of His Own
Episode 3: Getting Ahead of Myself
Episode 4: Outside the Box
Episode 5: Unwritten Rules
Episode 6: He Who Wins Shall Lose

Click for full description (may include spoilers)