The Rules: Don't take too long to think about it. Fifteen authors (poets included) who've influenced you and that will always stick with you. List the first 15 you can recall in no more than 15 minutes.
William Gibson - all his work prior to the late 90's or so
Neal Stephenson - everything
China Mieville - mostly everything
Orson Scott Card - Ender's Game
Warren Ellis - specifically Transmetropolitan but most of his stuff is brain sticky
Jean Craighead George - My Side of the Mountain
J.R.R. Tolkien - yeah.
Harlan Ellison - mostly his short stories
Edward Abbey - Desert Solitaire
David Brin - The Uplift books
Larry Niven - Protector, Ringworld, The Mote in God's Eye, The Integral Trees, everything else he's written
Mary Doria Russell - The Sparrow
Laurens van der Post - A Story Like The Wind, A Far Off Place
Stanislaw Lem - new to me, loving his stuff
Bill Waterson - Calvin and Hobbes
Bonus: whoever wrote the hacker's manifesto.
Robert Heinlein - can't believe I missed this one.
Frank Herbert - also shocking that i didn't think of him. Obviously need to re-read some classics.
Ursula K. Le Guin - Earthsea
Iain M. Banks