A couple of sites containing free sci-fi e-books:
Feedbooks site Feedbooks.com
Many sci-fi books, including those written by Cory Doctorow. File formats include ePUB, Kindle, PDF and "custom PDF". (You can also check out the whole site, Feedbooks.com for more public domain e-books.)
Baen library Baen.com
There quite a few books there, also some are fantasy ones. Many file formats available, e.g. HTML, RTF, ePUB, and those for mobile phones.
