The Collaborative International Dictionary of English v.0.48:

Guileless \Guile"less\, a.
   Free from guile; artless. -- Guile"less*ly, adv.
   Guile"less*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

guileless
    adj 1: free of deceit [syn: guileless, transparent]

Moby Thesaurus II by Grady Ward, 1.0:

63 Moby Thesaurus words for "guileless":
   artless, bluff, blunt, born yesterday, candid, childlike,
   confiding, credulous, dependent, depending, direct, frank, ingenu,
   ingenuous, innocent, naive, open, openhearted, outspoken, plain,
   reliant, relying, simple, simplehearted, simpleminded, sincere,
   single-hearted, single-minded, trustful, trusting, trusty,
   unaffected, unassumed, unassuming, unbeguileful, unbeguiling,
   uncalculating, undeceitful, undeceiving, undeceptive, undesigning,
   undisguised, undisguising, undissembling, undissimulated,
   undissimulating, unfeigned, unfeigning, unflattering, unguarded,
   unpretended, unpretending, unreserved, unschooled, unsophisticated,
   unstudied, unsuspecting, unsuspicious, untrimmed, untutored,
   unvarnished, unwary, without suspicion