The fic we've been waiting for with bated breath

Or maybe not so bated. I can't speak for all of us here.

A while ago Rowling wrote some 800-words for charity, and ended it with 'From the prequel I am not working on, but that was fun.'

Waterstone put up the works of the writers involved here. Click on 'Read our authors' stories'. It's a fun little read, even with her rather unclear handwriting. James and Sirius, and weird children's names.
(In case you really can't decipher her handwriting, it's transcribed here.)

Be sure to read Neil Gaiman's as well. It's shorter than Rowling's, but silly. I liked it.