WordNet (r) 3.0 (2006):

uncluttered
    adj 1: having nothing extraneous; "an uncluttered room"; "the
           unlittered shoulders of the road" [syn: uncluttered,
           unlittered]

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "uncluttered":
   austere, bare, basic, chaste, chipper, elementary, essential,
   fundamental, groomed, homely, homespun, homogeneous, indivisible,
   irreducible, mere, monolithic, neat, of a piece, orderly, plain,
   primal, primary, pure, pure and simple, severe, shipshape,
   simon-pure, simple, single, snug, spare, spick-and-span, stark,
   tidy, trig, trim, unadorned, undifferenced, undifferentiated,
   uniform, well