lots of useful posts above but
not sci fi so much but Project Gutenberg has lots of old stuff, by that I mean Dickens, Joseph Konrad etal
my gfs currently reading Wilde's "The Picture of Dorian Gray" from there and loves it.
but for Sci Fi there's Wells etc
https://www.gutenberg.org/ebooks/search/?query=h.+g.+wells
and they do have a science fiction category