The Collaborative International Dictionary of English v.0.48:

Ranter \Rant"er\ (r[a^]nt"[~e]r), n.
   1. A noisy talker; a raving declaimer.
      [1913 Webster]

   2. (Eccl. Hist.)
      (a) One of a religious sect which sprung up in 1645; --
          called also Seekers. See Seeker.
      (b) One of the Primitive Methodists, who seceded from the
          Wesleyan Methodists on the ground of their deficiency
          in fervor and zeal; -- so called in contempt.
          [1913 Webster]

WordNet (r) 3.0 (2006):

ranter
    n 1: someone who rants and raves; speaks in a violent or loud
         manner [syn: ranter, raver]