I just put my first batch of free ebooks up at the following page:
Free Sci-Fi ebooks
The first batch consists of Edgar Rice Burroughs first 5 John Carter of Mars books. They are freely downloadable in Mobipocket, html or pdf formats. If you've never read the John Carter stories, think of a cross between Tarzan, Buck Rogers, and Alan Quartermain. Remember they were written in the '20s, so the science is pretty absurd, but for raw adventure there's not much better.