The Collaborative International Dictionary of English v.0.48:

Unobtrusive \Un`ob*tru"sive\, a.
   Not obtrusive; not presuming; modest. -- Un`ob*tru"sive*ly,
   adv. -- Un`ob*tru"sive*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unobtrusive
    adj 1: not obtrusive or undesirably noticeable; "a quiet,
           unobtrusive life of self-denial" [syn: unobtrusive,
           unnoticeable] [ant: noticeable, obtrusive]

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "unobtrusive":
   Attic, aesthetic, artistic, back-door, backstairs, chaste, choice,
   clandestine, classic, covert, excellent, feline, furtive, hidlings,
   hole-and-corner, hugger-mugger, humble, in good taste, meek,
   modest, of choice, of quality, pleasing, privy, pure, quiet,
   restrained, shifty, simple, skulking, slinking, slinky, sly,
   sneaking, sneaky, stealthy, subdued, surreptitious, tasteful,
   tasty, unaffected, unambitious, unaspiring, unassuming, unboastful,
   under-the-counter, under-the-table, undercover, underground,
   underhand, underhanded, understated, unimposing, unostentatious,
   unpresuming, unpresumptuous, unpretending, unpretentious,
   well-chosen