(Note: sites are organized chronologically, so the top link is the latest one added.)
SF Site bills itself as "the home page for science fiction and fantasy," and that's a very good description. You'll find lots of book reviews here, as well as feature articles, commentary, interviews, and links to top SF magazines like Analog and Asmiov's. Go visit.
Spicy Green Iguana is a speculative fiction magazine link resource guide for writers, readers, and artists. It has more than 300 links to genre magazines large and small, print and online, and includes submission addresses, monthly reviews, and a section devoted to magazines without an Internet presence.
Critters Workshop is an on-line workshop/critique group for serious writers of science fiction, fantasy, and horror.
Abyss of Despair is a Web site that's also an interactive narrative. You can explore it or add your own areas.
AlphaDRIVE is a visually and aurally rich online magazine of speculative and futuristic works.
Speculative Vision is "the Science Fiction and Fantasy Resource Network" and is a nice place to spend some time.
Nuketown. Nuketown is a very well done Webzine of speculative fiction. You can also find movie reviews, X-Files info, and more.
SFF Net. "SFF Net is designed to support fast-moving, intelligent conversation about genre literature of all kinds. It's the place for authors, editors, readers, and publishers to get together to discuss books, stories, the art and craft of popular fiction, and all aspects of the literary life."
Clocktower Fiction. "Clocktower Fiction provides free quality original fiction for avid readers..."