WordNet (r) 3.0 (2006):

unprompted
    adj 1: proceeding from natural feeling or impulse without
           external stimulus; "an impulsive gesture of affection"
           [syn: impulsive, unprompted]

Moby Thesaurus II by Grady Ward, 1.0:

53 Moby Thesaurus words for "unprompted":
   arbitrary, automatic, autonomous, discretional, discretionary,
   elective, free, free will, gratuitous, impulsive, independent,
   instinctive, involuntary, nonmandatory, offered, optional,
   proffered, self-acting, self-active, self-determined,
   self-determining, spontaneous, unasked, unbesought, unbidden,
   uncalled-for, uncoerced, uncompelled, unconscious, undesigned,
   unforced, unguarded, unguided, uninfluenced, unintended,
   unintentional, uninvited, unmeant, unmeditated, unplanned,
   unpremeditated, unpressured, unpurposed, unrequested, unrequired,
   unsolicited, unsought, unthinking, unthought-of, unwitting,
   voluntary, volunteer, willful