WordNet (r) 3.0 (2006):

effortlessly
    adv 1: without effort or apparent effort; "she danced gracefully
           and effortlessly"; "swallows gliding effortlessly through
           the air"

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "effortlessly":
   adeptly, adroitly, easily, efficiently, expertly, facilely, freely,
   hands down, lightly, like clockwork, like nothing, no sweat,
   on easy terms, proficiently, readily, simply, skillfully, smoothly,
   swimmingly, well, without difficulty