The Collaborative International Dictionary of English v.0.48:

Effortless \Ef"fort*less\, a.
   1. Making no effort. --Southey.
      [1913 Webster]

   2. requiring little or no effort; as, an effortless victory.
      [PJC]

WordNet (r) 3.0 (2006):

effortless
    adj 1: requiring or apparently requiring no effort; "the
           swallows glided in an effortless way through the busy
           air" [ant: effortful]
    2: not showing effort or strain; "a difficult feat performed
       with casual mastery"; "careless grace" [syn: casual,
       effortless]

Moby Thesaurus II by Grady Ward, 1.0:

32 Moby Thesaurus words for "effortless":
   Mickey Mouse, adept, casual, clear, cursive, cushy, easy,
   easy as pie, expert, facile, flowing, fluent, glib, light,
   masterly, nothing to it, painless, plain, proficient, ready, royal,
   running, simple, simple as ABC, skilled, skillful, smooth, soft,
   straightforward, unburdensome, uncomplicated, untroublesome