The Collaborative International Dictionary of English v.0.48:

Ruminator \Ru"mi*na`tor\, n. [L.]
   One who ruminates or muses; a meditator.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ruminator
    n 1: a reflective thinker characterized by quiet contemplation
         [syn: muser, muller, ponderer, ruminator]