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.
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            Nature, informer of the poet's art.   --Pope.
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   2. One who informs, or imparts knowledge or news.
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   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.
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WordNet (r) 3.0 (2006):

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