If you like them _ultra short_ then I'd recommend 365tomorrows.com which feels like a single page ripped out of a greater (non-existent) work. Most, but not all, have a sci-fi theme.
I was surprised not to see Forster's stories as they are marvelous and public domain. People often cite the machine stops, but I recommend reading them all. The point of it and co-ordination are two favorites