Well, aside from Dune by Frank Herbert, there are, of course, many other great introductory novels to Science Fiction, Fantasy, and Speculative Fiction in general.
Novels from my own experience include Count Zero by William Gibson, Ender's Game by Orson Scott Card, Starship Troopers and The Moon is a Harsh Mistress by Robert A. Heinlein, The Mote in God's Eye by Larry Niven and Jerry Pournelle, Ringworld by Larry Niven, Lord of Light by Roger Zelazny and The Stars My Destination by Alfred Bester.
Some friends have recommended Brave New World by Aldous Huxley, and 1984 by George Orwell.
For fantasy, there's of course, The Lord of the Rings by J.R.R. Tolkien, The King of Elfland's Daughter by Lord Dunsany, Nine Princes in Amber by Roger Zelazny, Shadow of the Torturer by Gene Wolf, Elric by Michael Moorcock, Ill-met in Lankhmar by Fritz Lieber.