The Collaborative International Dictionary of English v.0.48:

Diligent \Dil"i*gent\, a. [F. diligent, L. diligens, -entis, p.
   pr. of diligere, dilectum, to esteem highly, prefer; di- =
   dis- + legere to choose. See Legend.]
   1. Prosecuted with careful attention and effort; careful;
      painstaking; not careless or negligent.
      [1913 Webster]

            The judges shall make diligent inquisition. --Deut.
                                                  xix. 18.
      [1913 Webster]

   2. Interestedly and perseveringly attentive; steady and
      earnest in application to a subject or pursuit; assiduous;
      industrious.
      [1913 Webster]

            Seest thou a man diligent in his business? he shall
            stand before kings.                   --Prov. xxii.
                                                  29.
      [1913 Webster]

            Diligent cultivation of elegant literature.
                                                  --Prescott.

   Syn: Active; assiduous; sedulous; laborious; persevering;
        attentive; industrious.
        [1913 Webster]

WordNet (r) 3.0 (2006):

diligent
    adj 1: quietly and steadily persevering especially in detail or
           exactness; "a diligent (or patient) worker"; "with
           persevering (or patient) industry she revived the failing
           business" [syn: diligent, persevering]
    2: characterized by care and perseverance in carrying out tasks;
       "a diligent detective investigates all clues"; "a diligent
       search of the files" [ant: negligent]

Moby Thesaurus II by Grady Ward, 1.0:

129 Moby Thesaurus words for "diligent":
   academic, advertent, agog, alert, all ears, all eyes, ardent,
   assiduous, attentive, aware, bookish, careful, concentrated,
   conscientious, conscious, constant, continuing, devoted to studies,
   dogged, donnish, dryasdust, earnest, elaborate, enduring,
   energetic, faithful, fervent, finical, finicking, finicky, hard,
   hard-working, hardworking, heedful, immutable, inalterable,
   indefatigable, indomitable, industrious, insistent, intense,
   intent, intentive, invincible, laborious, lasting, loyal, mandarin,
   meticulous, mindful, never idle, never-tiring, nice, niggling,
   observant, observing, obstinate, on the ball, on the job,
   open-eared, open-eyed, openmouthed, operose, owlish, painstaking,
   patient, patient as Job, pedantic, permanent, perseverant,
   persevering, persistent, persisting, pertinacious, plodding,
   plugging, preoccupied, professorial, punctilious, rabbinic, rapt,
   regardful, relentless, resolute, scholarly, scholastic, scrupulous,
   sedulous, single-minded, sleepless, slogging, stable, steadfast,
   steady, strenuous, stubborn, studious, 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