I got an odd blast-from-the-past-y feeling when SF Signal mentioned that Brain Twister, The Impossibles, and Supermind were all available from ManyBooks. I have these three books, with those covers and everything, about three feet away from me, as I stole borrowed them from my dad when I moved out. (Hi, Daddy! I have your books!)
This site is a fantastic resource for free e-texts, and is just another example of the power of the Internet; it’s maintained by Some Guy, as a public service. I just love that.
(Note: This is hardly the only game in town for free SF online. Google “free science fiction ebooks” for enough to keep you reading until your eyeballs fall out.)