The Collaborative International Dictionary of English v.0.48:

Experimenter \Ex*per"i*men`ter\, n.
   One who makes experiments; one skilled in experiments. --
   Faraday.
   [1913 Webster]

WordNet (r) 3.0 (2006):

experimenter
    n 1: a research worker who conducts experiments
    2: a person who enjoys testing innovative ideas; "she was an
       experimenter in new forms of poetry"