For some lighter sci-fi reading, I highly reccommend Spider Robinson's Callahan's Crosstime Saloon series. There are several books... 7 or 8 in the series now, I think. They most certainly must be read in order.These are some of the better books I've read in terms of creating characters that you can care about, and the books really do leave you with something.
William Gibson's Neuromancer is also rather good.
For fantasy, David B. Coe's Lonn/Tobyn Chrinicles is one of the better modern fantasy trilogies I've read in a long time.
The Retrieval Artist books by Kristine Kathryn Rusch are pretty good, and Robert Charles Wilson has an interesting one out called The Chronoliths.
If you really want to get into some deep stuff, then some of the best fantasy literature I've ever read was Dante's Commedy (John Cardi does a very nice translation).
Caleb Carr has a horribly written book, Killing Time that's worth the read anyway, thanks to the interesting ideas it presents.
Someone suggested Piers Anthony earlier.... I agree, if you're talking about his earlier works, and even some of the early Xanth novels weren't too horribly bad (Ok, the first 2 were tolerable).
Oh, also, on a light-hearted fun note, more children's sci-fi really, but still an entertaining little read are Janet Asimov's Norby books.