H.G. Wells - "The Time Machine", "War of The Worlds", "The Island of Dr. Moreau". FANTASTIC page-turning classic Victorian science fiction. The Island of Dr. Moreau is actually my personal favorite of his - I read it all in one sitting.
Jules Verne - "20,000 Leagues Under the Sea", "Around the World in Eighty Days", etc. More classic Victorian sci-fi. Where Wells focused on the "fiction", Verne tended to focus on the "science", so his stuff can be a tougher read but he made some astonishingly mindblowing predictions about the future.
Isaac Asimov - "I, Robot" series, "Foundation" series (esp. the original Foundation Trilogy) - I don't even know where to begin with how good this stuff is, Foundation is just a brilliant series and some of the story twists are just magical. Also READ HIS SHORT STORIES THEY'RE SO GOOD ASL;KGFHA;SELTR
Robert A. Heinlein - he has a lot of stuff but read "Starship Troopers"; if you can get past his weird author monologues about spanking kids this is a real jewel of a book that brings a real human element to the idea of blasting bugs.
Phillip K. Dick - "Do Androids Dream of Electric Sheep" - turned into Blade Runner. Recommended.
William Gibson - "Neuromancer" - THE ultimate cyberpunk story and probably one of the greatest in that genre, if not THE greatest. A real mind-bender of a read.
Stanislaw Lem - "Solaris" - A neato read that makes the alien, well, alien. This guy also has some brilliant short stories.
And, of course:
Douglas Adams - "The Hitchhiker's Guide to the Galaxy" - quite possibly the funniest book I've ever read.
I'm sure later I'll be kicking myself for forgetting somebody but hopefully Mister Adequate can pick up my slack