The Collaborative International Dictionary of English v.0.48:

Energetic \En`er*get"ic\, Energetical \En`er*get"ic*al\, a. [Gr.
   ?, fr. ? to work, be active, fr. ? active. See Energy.]
   1. Having energy or energies; possessing a capacity for
      vigorous action or for exerting force; active. "A Being
      eternally energetic." --Grew.
      [1913 Webster]

   2. Exhibiting energy; operating with force, vigor, and
      effect; forcible; powerful; efficacious; as, energetic
      measures; energetic laws.

   Syn: Forcible; powerful; efficacious; potent; vigorous;
        effective; strenuous. -- En`er*get"ic*al*ly, adv. --
        En`er*get"ic*al*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

energetic
    adj 1: possessing or exerting or displaying energy; "an
           energetic fund raiser for the college"; "an energetic
           group of hikers"; "it caused an energetic chemical
           reaction" [ant: lethargic, unenrgetic]
    2: working hard to promote an enterprise [syn: energetic,
       gumptious, industrious, up-and-coming]

Moby Thesaurus II by Grady Ward, 1.0:

110 Moby Thesaurus words for "energetic":
   active, activist, activistic, acute, aggressive, alive, animated,
   ardent, armipotent, assiduous, authoritative, bouncing, bouncy,
   breezy, brisk, bubbly, chipper, cogent, diligent, dynamic,
   ebullient, effective, effervescent, enterprising, enthusiastic,
   fervent, forceful, forcible, frisky, full of beans, full of go,
   full of life, full of pep, go-go, hard, hardworking, hearty,
   high-potency, high-powered, high-pressure, high-tension, hot,
   impetuous, in force, in power, incisive, indefatigable,
   industrious, intense, invigorated, irresistible, keen, kinetic,
   laborious, live, lively, living, lusty, mercurial, mettlesome,
   mighty, mighty in battle, militant, never idle, operative, peppy,
   perky, pert, potent, powerful, prepotent, puissant, quicksilver,
   relentless, robust, ruling, sedulous, sleepless, smacking, snappy,
   spanking, spirited, sprightly, spry, strenuous, striking, strong,
   take-charge, take-over, telling, tireless, trenchant, unflagging,
   unremitting, unsleeping, unsparing, untiring, unwearied, valid,
   vehement, vibrant, vigorous, vital, vivacious, vivid, zealous,
   zestful, zesty, zingy, zippy