Ender's Game is a good read.
Rendezvous with Rama is also good (but quite old).
Orson Scott Card's PastWatch was very interesting, a unique take on time travel.
Neil Stephenson's Snow Crash is cool if you like Cyberpunk.
Boneshaker has been recommended to me a couple times, I need to check it out. It's a steam punk novel. Not sure how that'd translate to fiction, but I'm curious anyway.
Completely unrelated to SciFi, but if you haven't read Freakonomics, it'll totally turn your head around about a lot of things.