All of the following are excellent scifi (author, followed by books).
Jules Verne - free online
Journey to the Center of the Earth (my favorite scifi)
20,000 Leagues Under the Sea
Mysterious Island
H G Wells - free online, and roughly half-length novels
War of the Worlds
The Time Machine (downer ending)
The Invisible Man (unlike many IM stories, he's a villain. This is the original)
Isaac Asimov
I, Robot (you *must* read this - the only "must read" scifi)
Elijah Baley books - http://en.wikipedia.org/wiki/Elijah_Baley excellent both as scifi *and* as murder mysteries - the best of both worlds (warning: adult content in the 3rd book)
Foundation Series - *the* classic scifi trilogy
Andre Norton - *the* classic female scifi author, most of her books involve "coming of age" and pure scifi adventure
http://en.wikipedia.org/wiki/Bibliography_of_Andre_Norton
her best: Star Man's Son (also titled 2250 A.D.)
other excellent:
Judgment on Janus
Forerunner series
Central Control series
Solar Queen series
The Time Traders (Atlantis, magic - very cool!)
The Zero Stone
You may also like Andre Norton's "Moon Called", a woman with psychic and magical powers comes of age in a post-nuclear holocaust world with - well, wait until you meet the *real* villains!
Another excellent: Mrs. Frisby and the Rats of NIMH. Though written for younger readers, I read this recently and it is great! Better than the movie (which was also pretty good).
Jim, http://www.life-after-harry-potter.com