Here is some online fiction that I thought was amusing, in one sense or another.

 

* = complete

 

Mother of Learning

A student in a Harry Potter style magic school gets involved with a time loop.

 

Worm*

In a world where people start gaining superpowers, this teenage girl can control insects. And she's up against the government.

 

Dungeon Keeper Ami

Sailor Mercury becomes a dungeon master.

 

Death March

A logical guy is reincarnated in a fantasy game world and has an easy time.

 

To The Stars

Far-future science fiction Madoka fanfiction where magical girls help starships fight aliens. Yep.

 

Luminosity*

Twilight fanfiction by someone from LessWrong where the main character is replaced by a LW-style Mary Sue.

 

HPMOR* (review)

Same as the above, except it's Harry Potter fanfiction.