The Collaborative International Dictionary of English v.0.48:

Inconspicuous \In`con*spic"u*ous\, a. [L. inconspicuus. See
   In- not, and Conspicuous.]
   Not conspicuous or noticeable; hardly discernible. --
   In`con*spic"u*ous*ly, adv. -- In`con*spic"u*ous*ness, n.
   --Boyle.
   [1913 Webster]

WordNet (r) 3.0 (2006):

inconspicuous
    adj 1: not prominent or readily noticeable; "he pushed the
           string through an inconspicuous hole"; "the invisible
           man" [syn: inconspicuous, invisible] [ant:
           conspicuous]

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "inconspicuous":
   blear, bleared, bleary, blurred, blurry, confused, dark, dim,
   discreet, faint, feeble, filmy, foggy, fuzzy, half-seen,
   half-visible, hazy, ill-defined, indefinite, indistinct,
   indistinguishable, insignificant, low-profile, merely glimpsed,
   misty, modest, obscure, out of focus, pale, semivisible, shadowy,
   unassuming, uncertain, unclear, undefined, undistinguished,
   unemphatic, unnoticeable, unnoticed, unobtrusive, unostentatious,
   unplain, unrecognizable, vague, weak