The Collaborative International Dictionary of English v.0.48:

Voluntarily \Vol"un*ta*ri*ly\, adv.
   In a voluntary manner; of one's own will; spontaneously.
   [1913 Webster]

WordNet (r) 3.0 (2006):

voluntarily
    adv 1: out of your own free will; "he voluntarily submitted to
           the fingerprinting" [ant: involuntarily]

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "voluntarily":
   advisedly, by choice, by design, calculatedly, consciously,
   contemplatedly, deliberately, designedly, electively, freely,
   gratis, gratuitously, in cold blood, independently, intentionally,
   knowingly, meditatedly, on purpose, optionally, pointedly,
   premeditatedly, purposefully, purposely, purposively,
   spontaneously, willfully, willingly, with full intent, with intent,
   with malice aforethought, with purpose, without coercion,
   wittingly