The Collaborative International Dictionary of English v.0.48:

Informant \In*form"ant\, n. [L. informans, -antis, p. pr. of
   informare. See Inform, v. t.]
   1. One who, or that which, informs, animates, or vivifies.
      [Obs.] --Glanvill.
      [1913 Webster]

   2. One who imparts information or instruction.
      [1913 Webster]

   3. One who offers an accusation; an informer. See Informer.
      [Obs. or R.]
      [1913 Webster]

            It was the last evidence of the kind; the informant
            was hanged.                           --Burke.
      [1913 Webster]

WordNet (r) 3.0 (2006):

informant
    n 1: a person who supplies information [syn: informant,
         source]
    2: someone who sees an event and reports what happened [syn:
       witness, witnesser, informant]

Moby Thesaurus II by Grady Ward, 1.0:

51 Moby Thesaurus words for "informant":
   adviser, announcer, annunciator, attestant, attestator, attester,
   authority, bystander, channel, cojuror, communicant, communicator,
   compurgator, deponent, earwitness, enlightener, examinant,
   examinate, examinee, expert witness, eyewitness, gossipmonger,
   grapevine, information center, information medium, informer,
   interviewee, monitor, mouthpiece, newsmonger, notifier, passerby,
   press, public relations officer, publisher, questionee, quizzee,
   radio, reporter, source, spectator, spokesman, subject, swearer,
   television, teller, testifier, tipster, tout, voucher, witness