WordNet (r) 3.0 (2006):

pardner
    n 1: an associate in an activity or endeavor or sphere of common
         interest; "the musician and the librettist were
         collaborators"; "sexual partners" [syn: collaborator,
         cooperator, partner, pardner]

Moby Thesaurus II by Grady Ward, 1.0:

68 Moby Thesaurus words for "pardner":
   ace, amigo, associate, bedfellow, bedmate, birthmate, bosom buddy,
   buddy, bunkie, bunkmate, butty, camarade, chamberfellow, chum,
   classmate, clubmate, colleague, comate, companion, company,
   compeer, comrade, confrere, consociate, consort, copartner,
   copemate, copesmate, couchmate, cradlemate, crony, cupmate, fellow,
   fellow student, general partner, girl friend, gossip, jailmate,
   mate, messmate, old crony, pal, pard, partner, pewmate, playfellow,
   playmate, roommate, schoolfellow, schoolmate, secret partner,
   shelfmate, shipmate, shopmate, side partner, sidekick, sidekicker,
   silent partner, sleeping partner, special partner, tablemate,
   teammate, tentmate, watchmate, waymate, workfellow, yokefellow,
   yokemate