The Collaborative International Dictionary of English v.0.48:

Informer \In*form"er\ ([i^]n*f[^o]rm"[~e]r), n. [From Inform,
   v.]
   1. One who informs, animates, or inspires. [Obs.] --Thomson.
      [1913 Webster]

            Nature, informer of the poet's art.   --Pope.
      [1913 Webster]

   2. One who informs, or imparts knowledge or news.
      [1913 Webster]

   3. (Law) One who informs a magistrate of violations of law;
      one who informs against another for violation of some law
      or penal statute.
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   Common informer (Law), one who habitually gives information
      of the violation of penal statutes, with a view to a
      prosecution therefor. --Bouvier. --Wharton.
      [1913 Webster]

WordNet (r) 3.0 (2006):

informer
    n 1: one who reveals confidential information in return for
         money [syn: informer, betrayer, rat, squealer,
         blabber]

Moby Thesaurus II by Grady Ward, 1.0:

114 Moby Thesaurus words for "informer":
   Benedict Arnold, Brutus, Judas, Judas Iscariot, Quisling, accusant,
   accuser, adviser, allegator, announcer, annunciator, appellant,
   archtraitor, attestant, attestator, attester, authority, betrayer,
   blab, blabber, blabberer, blabbermouth, bystander, canary, channel,
   claimant, cockatrice, cojuror, communicant, communicator,
   complainant, compurgator, conniver, conspirator, conspirer,
   delator, deponent, double agent, double-crosser, double-dealer,
   earwitness, enlightener, expert witness, eyewitness, fink,
   gossipmonger, grapevine, grass, impeacher, impugner, indictor,
   informant, information center, information medium, interviewee,
   intrigant, intriguer, libelant, machinator, monitor, mouthpiece,
   narc, nark, newsmonger, notifier, party, passerby, peacher,
   petitioner, plaintiff, plotter, press, prosecutor,
   public relations officer, publisher, quisling, radio, rat,
   reporter, schemer, serpent, snake, snitch, snitcher, source,
   spectator, spokesman, spy, squealer, stool, stool pigeon, stoolie,
   suitor, swearer, talebearer, taleteller, tattler, tattletale,
   television, teller, telltale, testifier, the prosecution,
   timeserver, tipster, tout, traitor, treasonist, trimmer, turncoat,
   voucher, weasel, whistle-blower, witness



Bouvier's Law Dictionary, Revised 6th Ed (1856):

INFORMER. A person who informs or prefers an accusation against another,
whom he suspects of the violation of some penal statute.
     2. When the informer is entitled to the penalty or part of the penalty,
upon the conviction of an offender, he is or is not a competent witness,
accordingly as the statute creating the penalty has or has not made him so.
1 Phil. Ev. 97; Rosc. Cr. Ev. 107; 5 Mass. R. 57; 1 Dall. 68; 1 Saund. 262,
c. Vide articles Prosecutor; Rewards.