Usefulness
I was browsing around in my IE favorites list and came across this little jewel: the Science Fiction Writers of America web page. It's actually a professional organization, kind of guild-ish, actually. Some of their chat transcripts are kind of weird, almost; reading your favorite authors chatting it up like *gasp* real people is a different experience. Their "Writing: The Craft" section is really good for anyone who writes science fiction, and useful even if you don't.