The Collaborative International Dictionary of English v.0.48:

Assiduous \As*sid"u*ous\, a. [L. assiduus, fr. assid?re to sit
   near or close; ad + sed[=e]re to sit. See Sit.]
   1. Constant in application or attention; devoted; attentive;
      unremitting.
      [1913 Webster]

            She grows more assiduous in her attendance.
                                                  --Addison.
      [1913 Webster]

   2. Performed with constant diligence or attention;
      unremitting; persistent; as, assiduous labor.
      [1913 Webster]

            To weary him with my assiduous cries. --Milton.
      [1913 Webster]

   Syn: Diligent; attentive; sedulous; unwearied; unintermitted;
        persevering; laborious; indefatigable.
        [1913 Webster] As*sid"u*ous*ly, adv. --
        As*sid"u*ous*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

assiduous
    adj 1: marked by care and persistent effort; "her assiduous
           attempts to learn French"; "assiduous research";
           "sedulous pursuit of legal and moral principles" [syn:
           assiduous, sedulous]

Moby Thesaurus II by Grady Ward, 1.0:

112 Moby Thesaurus words for "assiduous":
   advertent, agog, alert, all ears, all eyes, ardent, attentive,
   aware, careful, concentrated, conscious, constant, continuing,
   diligent, dogged, earnest, elaborate, enduring, energetic,
   faithful, fervent, finical, finicking, finicky, hard, hardworking,
   heedful, immutable, inalterable, indefatigable, indomitable,
   industrious, insistent, intense, intent, intentive, invincible,
   laborious, lasting, loyal, meticulous, mindful, never idle,
   never-tiring, nice, niggling, observant, observing, obstinate,
   on the ball, on the job, open-eared, open-eyed, openmouthed,
   operose, painstaking, patient, patient as Job, permanent,
   perseverant, persevering, persistent, persisting, pertinacious,
   plodding, plugging, preoccupied, rapt, regardful, relentless,
   resolute, sedulous, single-minded, sleepless, slogging, stable,
   steadfast, steady, strenuous, stubborn, tenacious, thorough,
   thoroughgoing, tireless, unabating, unconquerable, undaunted,
   undiscouraged, undrooping, unfailing, unfaltering, unflagging,
   unflinching, unintermitting, uninterrupted, unnodding, unrelaxing,
   unrelenting, unremitting, unsleeping, unsparing, unswerving,
   untiring, unwavering, unwearied, unwearying, unwinking,
   utterly attentive, vehement, watchful, weariless, zealous