The Collaborative International Dictionary of English v.0.48:

Tireless \Tire"less\, a.
   Untiring.
   [1913 Webster]

WordNet (r) 3.0 (2006):

tireless
    adj 1: showing sustained enthusiastic action with unflagging
           vitality; "an indefatigable advocate of equal rights"; "a
           tireless worker"; "unflagging pursuit of excellence"
           [syn: indefatigable, tireless, unflagging,
           unwearying]
    2: characterized by hard work and perseverance [syn:
       hardworking, industrious, tireless, untiring]

Moby Thesaurus II by Grady Ward, 1.0:

88 Moby Thesaurus words for "tireless":
   active, ardent, assiduous, constant, continuing, determined,
   diligent, dogged, dynamic, enduring, energetic, enthusiastic,
   faithful, fervent, hard, hardworking, immutable, inalterable,
   indefatigable, indomitable, industrious, inexhaustible, insistent,
   invincible, laborious, lasting, lively, loyal, never idle,
   never-tiring, obstinate, patient, patient as Job, permanent,
   perseverant, persevering, persistent, persisting, pertinacious,
   plodding, plugging, preoccupied, rapt, relentless, resolute,
   sagging, sedulous, single-minded, sleepless, slogging, spirited,
   stable, staunch, steadfast, steady, strenuous, stubborn, tenacious,
   unabating, unconquerable, undaunted, undeviating, undiscouraged,
   undrooping, unfailing, unfaltering, unflagging, unflinching,
   unintermitting, uninterrupted, unnodding, unrelaxing, unrelenting,
   unremitting, unsleeping, unsparing, unswerving, untiring,
   unwavering, unwearied, unwearying, unwinking, utterly attentive,
   vehement, vigorous, vital, weariless, zealous