(This whole page is sadly out-of-date, and will be made more practical Real Soon Now... -- JAF)
A short segment from Crash by J.G. Ballard
Poppy Z. Brite has posted a segment of her thord novel, Exquisite Corpse
Debra Doyle and James D. MacDonald maintain an extensive Web site (well worth visiting for all its resources) which includes a bibliography with links to excerpts of their writing.
Christa Faust has posted (appropriately enough) Chapter 13 of her horror novel, Control Freak.
Maureen F. McHugh maintains her own Web site with several essays, stories, and a sample from China Mountain Zhang.
An excerpt from Fred Saberhagen's A Sharpness on the Neck
Not sf, but excellent non-fiction by an sf writer: The Hacker Crackdown by Bruce Sterling.
Tor Books has an excellent Web site maintained by Patrick Nielsen Hayden & Teresa Nielsen Hayden. They offer a multitude of samples and chapters from many of science fiction's finest writers. For that matter, you may wish to check out their entire site starting with their homepage.
TWAIN is HotWired's literary online zine, occasionally featuring speculative fiction. You may need to become a member of this service, but there is no cost nor strings attached.