(A very partial list)
Ray Bradbury is, of course, a master, the most brilliant short story writer I've ever read, and one of the few writers still capable of making me feel awe. If you've never read him, shame.
I first found Earth on a library bookshelf when I was 12. Since then David Brin has been a favorite author of mine - always fun, and always with new and exciting ideas. In addition I spent several years as a teen hanging out on his listserve, Brin-L, and can testify that he's also very kind to young idiots.
The Bujold Nexus is the fan-run homepage of Lois McMaster Bujold, one of the funniest and best writers out there right now.
Tim Cockey writes the marvelous and witty Hitchcock Sewell mysteries. He was also the best man at my parent's wedding. My father likes to insist that the two are related.
Holly Lisle writes excellent fantasy novels, and has moreover sunk an astounding amount of her time and money into helping newer, younger writers get started. Her site has great writing (including some free novels) and great articles if you're a writer.
Ursula K. LeGuin is one of those authors that I cannot talk about with anything resembling objectivity. I've been reading her since I was very young and she's a major reason I want to be a writer. Don't let that put you off her, however - she's brilliant too.
Strange Horizons is a weekly webzine with some great, great, great stuff - for free.
Tad Williams is pretty well known to anyone who's read fantasy as the propigator of really, really big books. And good ones. Shadowmarch is an online writing project he once tried, now to be published as regular books by DAW, and still a thriving community of Williams fans.