What is a good science fiction short story?

Some of my favorites that you can read online:

All Summer in a Day by Ray Bradbury

http://www.wssb.org/content/Classrooms/tate/conten...

There Will Come Soft Rains by Ray Bradbury

http://www.gs.cidsnet.de/englisch-online/originals...

Examination Day by Henry Slesar

http://www.thebostonbachelor.com/2008/examination-...

Harrison Bergeron by Kurt Vonnegut

http://www.tnellen.com/cybereng/harrison.html

Here are a handful that I could recommend:

Young Zaphod Plays it Safe - Douglas Adams

Bicentennial Man - Isaac Asimov

Satisfaction Guaranteed - Isaac Asimov

Insert Knob A In Hole B - Isaac Asimov

I Have No Mouth And I Must Scream - Harlan Ellison

"Repent, Harlequin!" Said the Ticktockman - Harlan Ellison

Soldier - Harlan Ellison

We Can Remember It For You Wholesale - Phillip K Dick

The Minority Report - Phillip K Dick

Unfortunately, there isn't really any short fiction work by Stan Lee or Gene Roddenberry outside of their respective claims to fame. Lee's work is mostly in the realm of comic books (aside from some TV shows and novels that were largely written or ghostwritten by other parties) and Roddenberry's writing was almost exclusively for TV (and, pre-Star Trek, was mostly in the area of crime dramas and westerns).

The Blue Bottle - Ray Bradbury

Hb Wells

'A Sound of Thunder' - Ray Bradbury.

It's basically all about the butterfly effect. Great story.