I'm a fan of SciFi and Fantasy myself. I'll contain my recommendations to SciFi for you.
First I want to second some recommendations I've already seen on this thread. Snow Crash by Neal Stephenson is a fabulous read! One of my faves. The Diamond Age didn't really do it for me, though... Also, Hyperion and the Fall of Hyperion by Dan Simmons. If you like those, read Endymion and The Rise of Endymion. They continue the story. Pretty good books. They made my bookshelf, as Keepers, whereas most books I read end up at the used book store when I'm done.
Let's see, what else made the Keeper shelf?
The Bug Wars by Robert Aspirin - An oldie but a goodie!
In Conquest Born by C.S. Friedman, as well as it's "sequel", The Wilding. Also, This Alien Shore, which I haven't read in forever. Might need to break that one out soon. C.S. Friedman is an excellent author. I'd give anything she wrote a try.
Last, there is a loosely based trilogy by China Mieville that might qualify as SciFi. I loved the first book, Perdido Street Station. The second book, The Scar, is one of my favorite books. The Third, Iron Council, was simply painful to read. Fortunately, they are all stand alone novels, so it wasn't like the end of one long story just sucked out loud...
I found that re-reading the Madeleine L'Engle books recently as an adult was thought-provoking. The neat thing about them is that they entertain younger readers but also contain higher level math and physics.
A Wrinkle in Time
A Wind in the Door
A Swiftly Tilting Planet
Many Waters
An Acceptable Time
She also has a number of other fiction and non-fiction works. I'm starting the Crosswicks Journal series and I've read her book Walking On Water - about being a writer and a woman of faith.
I also couldn't get through Eragon. Boring!!!! For great kid fantasy fiction, not previously mentioned, the Perseus Jackson series by Rick Riordan is wonderful. The Greek myths are real, alive and living in the US. Everyone in my house old enough to read them inhaled and read them as fast as possible....not patiently waiting for the next in the series.
I also couldn't get through Eragon. Boring!!!! For great kid fantasy fiction, not previously mentioned, the Perseus Jackson series by Rick Riordan is wonderful. The Greek myths are real, alive and living in the US. Everyone in my house old enough to read them inhaled and read them as fast as possible....not patiently waiting for the next in the series.