One of the premiere SF webzines was Event Horizon, an early publisher of stories by the likes of Kelly Link, Jeffrey Ford, and Severna Park, as well as fiction by long-established pros like Gardner Dozois and Barry Malzberg. The site also featured nonfiction by Lucius Shepard, Jack Womack, Douglas Winter, and other names you might recognize.
Thanks to a comment in the previous post from Ellen Datlow, the editor of Event Horizon, I discovered that even though the old web address no longer works, the content is still entirely available via the Wayback Machine.
I had no luck getting past the first page with the Safari browser on a Mac, but after patient assurances from Ellen that, indeed, the entire content was really there, I switched over to Firefox, and, lo and behold, there it was.
A good way to see immediately what joys are available is through the site map. For reference, here are all the archived pages.