Project Gutenberg, which makes public-domain works freely available in electronic format, has a large selection of classic science fiction that has either fallen out of copyright due to age or whose copyright was not renewed, allowing it to move into public domain.
Branko Collin writes at TeleRead:
Authors include H. Beam Piper, Andre Norton, Terry Gene Carr, Ray Cummings, Lester Del Rey, Murray Leinster, Carey Rockwell, and more. You’ll discover a completer list with links at The Thunder Child, a web magazine for Sci-Fi and Fantasy, named after the famous ironclad from Wells’ War of the Worlds.
Among the works: an extensive lists of Tom Swift books, for those nostalgic for the boy techno-wiz.