Graham Sleight writes, "The third edition of the Encyclopedia of Science Fiction, the definitive reference work in the field, will be published online beginning later this year. Thanks to support from publishing partner Gollancz, the text will be available free. The Encyclopedia is overseen by editors John Clute and David Langford, with help from Editor Emeritus Peter Nicholls, Managing Editor Graham Sleight and a range of specialist editors from across the field. About 75% of the text will be online by the end of the year, with the rest being added in monthly updates to the end of 2012. Interested users can sign up at the website or follow us on twitter at @SFEncyclopedia."
SFE: The Encyclopedia of Science Fiction (Thanks, Graham!)
(Disclosure: I have done some free consulting for the SFE)