The Collaborative International Dictionary of English v.0.48:

Grader \Grad"er\, n.
   1. One who grades, or that by means of which grading is done
      or facilitated.

   2. Specifically: A vehicle used for levelling earth, esp. one
      with a plow blade suspended from the center, used
      specifically for grading roads.
      [PJC]

WordNet (r) 3.0 (2006):

grader
    n 1: a judge who assigns grades to something