The Collaborative International Dictionary of English v.0.48:

Harmless \Harm"less\ (h[aum]rm"l[e^]s), a.
   1. Free from harm; unhurt; as, to give bond to save another
      harmless.
      [1913 Webster]

   2. Free from power or disposition to harm; innocent;
      inoffensive. " The harmless deer." --Drayton

   Syn: Innocent; innoxious; innocuous; inoffensive;
        unoffending; unhurt; uninjured; unharmed.
        --Harm"less*ly, adv.- Harm"less*ness, n. Harmonic

WordNet (r) 3.0 (2006):

harmless
    adj 1: not causing or capable of causing harm; "harmless
           bacteria"; "rendered the bomb harmless" [ant: harmful]

Moby Thesaurus II by Grady Ward, 1.0:

67 Moby Thesaurus words for "harmless":
   benign, bright, dependable, fail-safe, firm, fresh, gentle,
   guaranteed, guiltless, innocent, innocuous, inoffensive, inviolate,
   invulnerable, mild, mint, nonmalignant, nonpoisonous, nontoxic,
   nonvirulent, painless, pristine, reliable, riskless, safe,
   scatheless, sound, stable, steady, trustworthy, unbroken,
   unbruised, undamaged, undamaging, undangerous, undefaced,
   undeformed, undemolished, undestroyed, unfaded, unharmed,
   unhazardous, unhurt, unhurtful, unimpaired, uninjured, uninjurious,
   unmaimed, unmangled, unmarked, unmarred, unobnoxious, unoffending,
   unperilous, unprecarious, unrisky, unscarred, unscathed,
   unscratched, unshattered, unspoiled, untorn, untouched, unwithered,
   unworn, virgin, warranteed