Project Gutenberg has a pretty good science fiction selection, quite extensive in fact that I think it's better to go by author than by individual works.
For the "classics" there's H.G. Wells and Edgar Rice Burroughs, aside from Verne and Shelley whom you've already mentioned.
There are some surprising names, too, like Jack London, E.M. Forster, and Rudyard Kipling.
For golden age scifi: Frederic Brown, E.E. "Doc" Smith, CM Kornbluth, Jack Williamson, Frederic Pohl, Olaf Stapledon, and Andre Norton. Also, Robert E. Howard and H.P. Lovecraft.
For your criteria, though, I would recommend looking for the works of Philip K. Dick and H. Beam Piper.