The Collaborative International Dictionary of English v.0.48:

Willful \Will"ful\, a. [Will + full.] [Written also wilful.]
   [1913 Webster]
   1. Of set purpose; self-determined; voluntary; as, willful
      murder. --Foxe.
      [1913 Webster]

            In willful poverty chose to lead his life.
                                                  --Chaucer.
      [1913 Webster]

            Thou to me
            Art all things under heaven, all places thou,
            Who, for my willful crime, art banished hence.
                                                  --Milton.
      [1913 Webster]

   2. Governed by the will without yielding to reason;
      obstinate; perverse; inflexible; stubborn; refractory; as,
      a willful man or horse.
      [1913 Webster] -- Will"ful*ly, adv. -- Will"ful*ness,
      n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

willful
    adj 1: done by design; "the insult was intentional"; "willful
           disobedience" [syn: willful, wilful]
    2: habitually disposed to disobedience and opposition [syn:
       froward, headstrong, self-willed, willful, wilful]

Moby Thesaurus II by Grady Ward, 1.0:

133 Moby Thesaurus words for "willful":
   advised, aimed, aimed at, arbitrary, autonomous, balking, balky,
   bigoted, bulldogged, bulletheaded, bullheaded, calculated,
   case-hardened, conscious, considered, contemplated, contumacious,
   decided, deliberate, deliberated, designed, determined,
   discretional, discretionary, disobedient, dogged, dogmatic,
   elective, envisaged, envisioned, factious, fanatic, free,
   free will, froward, gratuitous, hardheaded, headstrong, heady,
   independent, indisciplined, insubordinate, intended, intentional,
   intolerant, intractable, irresponsible, knowing, lawless,
   licentious, meant, meditated, mulish, mutinous, naughty,
   nonconforming, nonmandatory, obstinate, of design, offered,
   opinionated, optional, overzealous, persevering, pertinacious,
   perverse, pigheaded, planned, proffered, projected, proposed,
   purposed, purposeful, purposive, rampant, recusant, reinless,
   resolved, restive, self-acting, self-active, self-determined,
   self-determining, self-willed, set, spontaneous, stiff-necked,
   strong-willed, strongheaded, stubborn, studied, sulky, sullen,
   teleological, tenacious, transgressive, unaccountable, unasked,
   unbesought, unbidden, unbridled, uncalled-for, unchecked,
   uncoerced, uncompelled, uncomplying, uncontrolled, uncooperative,
   uncurbed, undisciplined, unduteous, undutiful, unforced,
   ungoverned, uninfluenced, uninvited, unpressured, unprompted,
   unregenerate, unreined, unrequested, unrequired, unrestrained,
   unsolicited, unsought, violative, voluntary, volunteer, wayward,
   wildcat, willing, witting, wrongheaded