The Collaborative International Dictionary of English v.0.48:

Thinker \Think"er\, n.
   One who thinks; especially and chiefly, one who thinks in a
   particular manner; as, a close thinker; a deep thinker; a
   coherent thinker.
   [1913 Webster]

WordNet (r) 3.0 (2006):

thinker
    n 1: an important intellectual; "the great minds of the 17th
         century" [syn: thinker, creative thinker, mind]
    2: someone who exercises the mind (usually in an effort to reach
       a decision)

Moby Thesaurus II by Grady Ward, 1.0:

58 Moby Thesaurus words for "thinker":
   Brahmin, Nestor, Solomon, authority, brainworker, casuist,
   cosmologist, dialectician, doctor, egghead, elder, elder statesman,
   expert, great soul, guru, highbrow, illuminate, intellect,
   intellectual, intellectualist, literate, logicaster, logician,
   logistician, lover of wisdom, mahatma, man of intellect,
   man of wisdom, mandarin, master, mastermind, mentor, metaphysician,
   oracle, philosophaster, philosophe, philosopher, philosophizer,
   pundit, rabbi, ratiocinator, rationalist, rationalizer, reasoner,
   rishi, sage, sapient, savant, scholar, seer, sophist, speculator,
   starets, syllogist, syllogizer, white-collar intellectual,
   wise man, wise old man