Creative Commons Science-Fiction on Feedbooks.com offers many ebooks published under Creative Commons license, as pioneered by Cory Doctorow.
The Encyclopedia of Science Fiction provides lots of interesting information and links.
Unz.org is a vast repository of books, articles, reviews, and videos in the public domain, searchable by author. It also provides links to help find works for purchase or in libraries.
LibriVox makes audiobooks available in the public domain.
Baen Books Free Library, providing samples of participating Baen authors’ works free, including complete books.
TOR Books offer free stories, art, and sample chapters from their upcoming books.
Edit: More suggestions received:
International Speculative Fiction, an online magazine that started last summer.
Lightspeed Magazine, another online science fiction and fantasy magazine.
Clarkesworld, a Hugo Award-winning and World Fantasy Award-nominated science fiction and fantasy magazine.
Escape Pod, a science fiction podcast magazine.
Pseudopod, short horror in audio form.
Podcastle, an audio fantasy magazine.
Daily SF, original science fiction and fantasy every weekday.
Strange Horizons, a free weekly online magazine devoted to publishing high-quality speculative fiction, poetry, art, and related nonfiction.
Starship Sofa, a Hugo-Award-winning science fiction podcast from the UK.
Worlds Without End is a science fiction, fantasy and horror and it includes a free books section.
Art by Julie Dillon, TOR Books. No copyright challenge intended.