In no particular order...
William Gibson - Haven't read his new stuff, but loved the original cyberspace 'trilogy'
Neal Stephenson - I haven't read as much of his work as I would like, but what I've read I enjoyed.
Alastair Reynolds - Rapidly becoming a favorite in terms of his short work - Need to read the Revelation Space series.
Iain M. Banks - Love, love, love the 'Culture' novels.
Richard K. Morgan - Big fan of the first two Takeshi Kovacs novels - although I still haven't gotten round to Woken Furies.
Isaac Asimov - Just a great writer who was incredibly lucid.
Harlan Ellison - Everything he writes is immensely readable.
Stephen R. Donaldson - His creation of the 'Land' in the Covenant novels is one of the greatest achievements in fantasy literature, IMHO.
Arthur C. Clarke - Love the old classics from Clarke.
Alfred Bester - Really ahead of his time.
Phillip K. Dick - A bit hit or miss, but I love some of his work and consider The Man in the High Castle one of the greatest SF novels ever written.
Lucius Shepard - Perhaps my favorite contemporary writer in terms of literary style.
Terry Pratchett - I had to edit this to add Mr. Pratchett. The Discworld novels are about as fun as it gets with regards to reading.
Edit: Oh, and I've definitely heard of
Clfford D. Simak, Wilson Tucker, Leigh Brackett, Roger Zelazny, and Frederic Brown.
- Walter...
disappointed that I was the only one to acknowledge our very own - HRUMPH 
