Home pages of magazines and book publishers. Many have guidelines.
Some have samples of the fiction they buy. It is most highly recommended
to read a magazine, to really know what they publish,
before you send in your stories.
Forums on, among other things, various aspects of writing. (Dan
Goodman recommends the one on writer's block.) Forums run by various
sff.net members -- who may include some of your favorite writers.
Links to writers' home pages.
Wonderfully informative site by Neil Weiner, and independent researcher
and writer, suumarizing historic background on everything from the
Balkans to the White House Staff. If everybody on TV is talking about
Forensic Evidence and you don't know what that is, check Neil Weiner.