The Collaborative International Dictionary of English v.0.48:

Ringer \Ring"er\, n.
   1. One who, or that which, rings; especially, one who rings
      chimes on bells.
      [1913 Webster]

   2. (Mining) A crowbar. --Simmonds.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Ringer \Ring"er\, n. (Horse Racing)
   A horse that is not entitled to take part in a race, but is
   fraudulently got into it.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ringer
    n 1: a person who rings church bells (as for summoning the
         congregation) [syn: toller, bell ringer, ringer]
    2: a person who is almost identical to another [syn: ringer,
       dead ringer, clone]
    3: a contestant entered in a competition under false pretenses
    4: (horseshoes) the successful throw of a horseshoe or quoit so
       as to encircle a stake or peg

Moby Thesaurus II by Grady Ward, 1.0:

83 Moby Thesaurus words for "ringer":
   agent, alternate, alternative, analogy, backup, blagueur, bluff,
   bluffer, change, changeling, charlatan, comparison, copy,
   counterfeit, deputy, double, dummy, equal, equivalent, ersatz,
   exchange, fake, faker, fill-in, fourflusher, fraud, ghost,
   ghostwriter, humbug, imitation, impersonator, impostor,
   locum tenens, makeshift, malingerer, metaphor, metonymy,
   mountebank, next best thing, personnel, phony, picture,
   pinch hitter, portrait, poser, poseur, pretender, proxy, quack,
   quacksalver, quackster, relief, replacement, representative,
   reserves, saltimbanco, second string, secondary, sham, shammer,
   sign, simulacrum, spares, spit, spitting image, stand-in, sub,
   substituent, substitute, substitution, succedaneum, superseder,
   supplanter, surrogate, symbol, synecdoche, third string, token,
   understudy, utility player, vicar, vice-president, vice-regent