The Collaborative International Dictionary of English v.0.48:

Petitioner \Pe*ti"tion*er\, n.
   One who presents a petition.
   [1913 Webster]

WordNet (r) 3.0 (2006):

petitioner
    n 1: one praying humbly for something; "a suppliant for her
         favors" [syn: petitioner, suppliant, supplicant,
         requester]
    2: someone who petitions a court for redress of a grievance or
       recovery of a right [syn: suer, petitioner]

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "petitioner":
   accusant, accuser, adorer, allegator, appellant, applicant, asker,
   aspirant, beadsman, beggar, bidder, candidate, celebrant,
   chapelgoer, churchgoer, claimant, communicant, complainant,
   congregation, daily communicant, delator, evangelist, idolater,
   impeacher, impugner, indictor, informer, libelant, party,
   plaintiff, postulant, prayer, prosecutor, revivalist, seeker,
   solicitant, solicitor, suitor, suppliant, supplicant, supplicator,
   the prosecution, venerator, votary, worshiper