The Collaborative International Dictionary of English v.0.48:

Ponderer \Pon"der*er\, n.
   One who ponders.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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