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]

Moby Thesaurus II by Grady Ward, 1.0:

62 Moby Thesaurus words for "ranter":
   Herod, Holy Willie, Joseph Surface, Mawworm, Pecksniff, Pharisee,
   Tartuffe, affecter, blatherskite, bluff, bluffer, blusterer,
   braggart, bravo, bucko, bully, bullyboy, canter, canting hypocrite,
   debater, declaimer, demagogue, dissembler, dissimulator, fanfaron,
   formalist, haranguer, hector, hectorer, hypocrite, jawsmith,
   lip server, lip worshiper, panelist, pharisee, pietist,
   pious fraud, poser, public speaker, rabble-rouser, raver,
   religionist, religious hypocrite, roisterer, sniveler, snuffler,
   speaker, speecher, speechifier, speechmaker, spieler,
   spiritual humbug, spokesman, spokeswoman, spouter, swaggerer,
   swashbuckler, swasher, talker, tub-thumper, vaporer,
   whited sepulcher