The Collaborative International Dictionary of English v.0.48:

Lifeless \Life"less\, a.
   Destitute of life, or deprived of life; not containing, or
   inhabited by, living beings or vegetation; dead, or
   apparently dead; spiritless; powerless; dull; as, a lifeless
   carcass; lifeless matter; a lifeless desert; a lifeless wine;
   a lifeless story. -- Life"less*ly, adv. --
   Life"less*ness, n.

   Syn: Dead; soulless; inanimate; torpid; inert; inactive;
        dull; heavy; unanimated; spiritless; frigid; pointless;
        vapid; flat; tasteless.

   Usage: Lifeless, Dull, Inanimate, Dead. In a moral
          sense, lifeless denotes a lack of vital energy;
          inanimate, a lack of expression as to any feeling that
          may be possessed; dull implies a torpor of soul which
          checks all mental activity; dead supposes a
          destitution of feeling. A person is said to be
          lifeless who has lost the spirits which he once had;
          he is said to be inanimate when he is naturally
          wanting in spirits; one is dull from an original
          deficiency of mental power; he who is dead to moral
          sentiment is wholly bereft of the highest attribute of
          his nature.
          [1913 Webster]

WordNet (r) 3.0 (2006):

lifeless
    adj 1: deprived of life; no longer living; "a lifeless body"
           [syn: lifeless, exanimate]
    2: destitute or having been emptied of life or living beings;
       "after the dance the littered and lifeless ballroom echoed
       hollowly"
    3: lacking animation or excitement or activity; "the party being
       dead we left early"; "it was a lifeless party until she
       arrived"
    4: not having the capacity to support life; "a lifeless planet"

Moby Thesaurus II by Grady Ward, 1.0:

191 Moby Thesaurus words for "lifeless":
   abeyant, abiotic, apathetic, arid, asleep, asleep in Jesus,
   at rest, azoic, bare, barren, benumbed, bereft of life, blah,
   blank, blase, bleak, bloodless, bored, boring, breathless,
   called home, carrion, cataleptic, catatonic, characterless, cold,
   colorless, croaked, dead, dead and gone, deadened, death-struck,
   debilitated, deceased, defunct, demised, departed,
   departed this life, desert, desolate, destitute of life, dismal,
   done for, dopey, dormant, drab, draggy, drearisome, dreary, droopy,
   drugged, dry, dryasdust, dull, dumb, dusty, effete, elephantine,
   empty, enervated, etiolated, exanimate, extinct, fade, fallen,
   finished, flat, food for worms, foul, gone, gone to glory,
   gone west, groggy, heavy, hebetudinous, ho-hum, hollow,
   in abeyance, in suspense, inactive, inane, inanimate, inanimated,
   inert, inexcitable, insensate, insensible, insentient, insipid,
   jaded, jejune, lackadaisical, lackluster, languid, languorous,
   late, late lamented, latent, launched into eternity, leaden,
   lethargic, listless, logy, low-spirited, lumpish, lusterless,
   martyred, mat, moribund, mute, no more, nonconscious, nonliving,
   numb, pale, pallid, passed on, passive, pedestrian, phlegmatic,
   plodding, pointless, poky, ponderous, pooped, prosaic, prosy,
   pushing up daisies, released, reposing, resting easy, sainted,
   sated, sedentary, senseless, slack, sleeping, sleepy, slow,
   sluggish, slumbering, smitten with death, smoldering, solemn,
   somber, somnolent, soulless, spiritless, stagnant, stagnating,
   stale, standing, static, sterile, stiff, still, stillborn, stodgy,
   stuffy, stultified, superficial, supine, suspended, taken away,
   taken off, tame, tasteless, tedious, tiresome, torpid, unanimated,
   unaroused, unconscious, unfeeling, uninhabited, uninteresting,
   unlively, unmoving, unoccupied, vapid, vegetable, vegetative, wan,
   waste, weary, with the Lord, with the saints, without life,
   without vital functions, wooden, world-weary