The Collaborative International Dictionary of English v.0.48:

Inquirer \In*quir"er\, n. [Written also enquirer.]
   One who inquires or examines; questioner; investigator.
   --Locke.
   [1913 Webster]

         Expert inquirers after truth.            --Cowper.
   [1913 Webster]

WordNet (r) 3.0 (2006):

inquirer
    n 1: someone who asks a question [syn: inquirer, enquirer,
         questioner, querier, asker]

Moby Thesaurus II by Grady Ward, 1.0:

54 Moby Thesaurus words for "inquirer":
   Paul Pry, Peeping Tom, asker, auditor, autodidact, busybody,
   catechist, cross-interrogator, cross-questioner, detective,
   eavesdropper, educatee, examiner, gossip, inquisitionist,
   inquisitive, inquisitor, interlocutor, interpellator, interrogator,
   interrogatrix, interviewer, learner, monitor, nosy Parker,
   opinion-sampler, poller, pollster, praepostor, prefect, prober,
   pry, pupil, querier, querist, questioner, questionist, quidnunc,
   quizzer, rubberneck, rubbernecker, sampler, scholar, scopophiliac,
   secret agent, self-taught person, sightseer, snoop, snooper,
   student, studier, trainee, voyeur, yenta