The Collaborative International Dictionary of English v.0.48:

Forceful \Force"ful\, a.
   Full of or processing force; exerting force; mighty. --
   Force"ful*ly, adv.
   [1913 Webster]

         Against the steed he threw
         His forceful spear.                      --Dryden.
   [1913 Webster]

WordNet (r) 3.0 (2006):

forceful
    adj 1: characterized by or full of force or strength (often but
           not necessarily physical); "a forceful speaker"; "a
           forceful personality"; "forceful measures"; "a forceful
           plan for peace" [ant: forceless, unforceful]
    2: forceful and definite in expression or action; "the document
       contained a particularly emphatic guarantee of religious
       liberty" [syn: emphatic, forceful]

Moby Thesaurus II by Grady Ward, 1.0:

147 Moby Thesaurus words for "forceful":
   accented, accentuated, acid, active, acute, adventuresome,
   adventurous, aggressive, ambitious, animated, armipotent,
   assertive, authoritative, beefy, biting, bouncing, brisk, cogent,
   compelling, constraining, convincing, corrosive, cutting, decided,
   doughty, driving, dynamic, effective, efficacious, emphasized,
   emphatic, energetic, enterprising, enthusiastic, forcible, forcy,
   full of pep, full-blooded, full-strength, go-ahead, go-go, gutsy,
   gutty, hale, hard, hard as nails, hardy, hearty, hefty,
   high-potency, high-powered, high-pressure, high-tension, husky,
   hustling, imperative, impetuous, impressive, in force, in power,
   in red letters, incisive, insistent, intense, iron-hard,
   irresistible, italicized, keen, kinetic, lively, living, logical,
   lusty, manful, meaty, mettlesome, mighty, mighty in battle,
   mordant, nervous, nervy, obstinate, operative, penetrating, peppy,
   persuasive, piercing, pithy, poignant, pointed, positive, potent,
   powerful, prepotent, puissant, punchy, punctuated, pushful,
   pushing, pushy, red-blooded, resounding, robust, robustious,
   rugged, ruling, sensational, sinewed, sinewy, slashing, smacking,
   snappy, spanking, spirited, stalwart, starred, steely, stout,
   strapping, strenuous, stressed, striking, strong, strong as brandy,
   strong as strong, strong-willed, sturdy, take-charge, take-over,
   telling, trenchant, underlined, underscored, up-and-coming, valid,
   venturesome, venturous, vibrant, vigorous, virile, vital,
   vivacious, vivid, weighty, zestful, zesty, zippy