The Collaborative International Dictionary of English v.0.48:

Unassuming \Un`as*sum"ing\, a.
   Not assuming; not bold or forward; not arrogant or presuming;
   humble; modest; retiring; as, an unassuming youth; unassuming
   manners.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unassuming
    adj 1: not arrogant or presuming; "unassuming to a fault,
           skeptical about the value of his work"; "a shy retiring
           girl" [syn: retiring, unassuming]

Moby Thesaurus II by Grady Ward, 1.0:

128 Moby Thesaurus words for "unassuming":
   Bohemian, affable, arcadian, authentic, bona fide, bucolic, candid,
   card-carrying, casual, cordial, degage, dinkum, direct, easy,
   easygoing, familiar, folksy, following the letter, free and easy,
   genuine, good, gracious, guileless, haymish, homely, homespun,
   homey, honest, honest-to-God, humble, inartificial, informal,
   irregular, lawful, legitimate, lifelike, literal, loose, lowly,
   meek, modest, native, natural, naturalistic, naturelike, offhand,
   offhanded, original, pastoral, plain, pure, real, realistic,
   relaxed, rightful, rural, simon-pure, simple, sincere, sociable,
   sterling, straightforward, sure-enough, true to life,
   true to nature, true to reality, unadorned, unadulterated,
   unaffected, unambitious, unartificial, unaspiring, unassumed,
   unbeguileful, unbeguiling, unboastful, uncalculating,
   unceremonious, uncolored, unconcocted, unconstrained,
   unconventional, uncopied, uncounterfeited, undeceitful,
   undeceiving, undeceptive, undesigning, undisguised, undisguising,
   undissembling, undissimulated, undissimulating, undistorted,
   unembellished, unexaggerated, unfabricated, unfanciful, unfeigned,
   unfeigning, unfictitious, unflattering, unimagined, unimitated,
   unimposing, uninvented, unobtrusive, unofficial, unostentatious,
   unpresuming, unpresumptuous, unpretended, unpretending,
   unpretentious, unqualified, unromantic, unsimulated, unspecious,
   unspoiled, unstudied, unsynthetic, untrimmed, unvarnished, verbal,
   verbatim, veridical, verisimilar, word-for-word