The Collaborative International Dictionary of English v.0.48:

Doubter \Doubt"er\, n.
   One who doubts; one whose opinion is unsettled; one who
   scruples.
   [1913 Webster]

WordNet (r) 3.0 (2006):

doubter
    n 1: someone who habitually doubts accepted beliefs [syn:
         skeptic, sceptic, doubter]
    2: someone who is doubtful or noncommittal about something [syn:
       agnostic, doubter]