Thought I'd mention a couple I didn't see mentioned here (though I'll toss out another I love Heinlein, because he's my favorite by far). I'm going purely off memory, so there may be some inaccuracies.
Lois McMaster Bujold: Vorsorkian series (all of them). Starts with Cordelia's Honor (IIRC). GREAT books, better than the Honor Harrington series IMO (if you like the Honor books, you'll love this series)
Christopher Stasheff: more a mix of sci-fi and fantasy, but close enough to include him. I personally enjoyed the Warlock series, but he also co-authored some books with L Sprague de Camp.
Spider Robinson: He's classifed as sci-fi, but I think it's a stretch. It's not *quite* fantasy either. So I think they've just lumped him in. I love the Callahan series, but 'God is an Iron' is great (book of short stories). If you like Heinlein, he's worth reading (though you'll be annoyed at first that he's dubbed Heinlein's successor...though you may find yourself later agreeing. He's also the world's biggest Heinlein fanboy which is saying a lot).
Harry Turtledove: His books are listed as alternative history, but frankly that should be under sci-fi with how he writes. Guns of the South was a GREAT book. His alternative WWII series is also fantastic. If you're a history buff, all of his stuff is great.
Harry Harrison: (I know someone mentioned him) but the Stainless Steel Rat series usually doesn't get a lot of notice. I grew up on those books, so I think they're fantastic. (He also wrote "Make Room! Make Room!" which became the movie Soylent Green. Not a great plug but funny).
Anne McCafferty: Pern series, only the early books. As it goes on and on and on, it kinda craps out. (IMO)
I'm sure I'll add more later as I'll think 'oh man I didn't mention....'

Also saw a few books posted I'll have to pick up, so thanks!
