The Collaborative International Dictionary of English v.0.48:

Supplemental \Sup`ple*men"tal\, Supplementary
\Sup`ple*men"ta*ry\, a. [Cf. F. suppl['e]mentaire.]
   Added to supply what is wanted; additional; being, or serving
   as, a supplement; as, a supplemental law; a supplementary
   sheet or volume.
   [1913 Webster]

   Supplemental air (Physiol.), the air which in addition to
      the residual air remains in the lungs after ordinary
      expiration, but which, unlike the residual air, can be
      expelled; reserve air.

   Supplemental bill (Equity), a bill filed in aid of an
      original bill to supply some deffect in the latter, or to
      set forth new facts which can not be done by amendment.
      --Burrill. --Daniel.

   Supplementary chords (Math.), in an ellipse or hyperbola,
      any two chords drawn through the extremities of a
      diameter, and intersecting on the curve.
      [1913 Webster]

WordNet (r) 3.0 (2006):

supplementary
    adj 1: functioning in a supporting capacity; "the main library
           and its auxiliary branches" [syn: auxiliary,
           subsidiary, supplemental, supplementary]
    2: added to complete or make up a deficiency; "produced
       supplementary volumes" [syn: supplementary, supplemental]

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "supplementary":
   accessory, accidental, added, additional, adjunct, adscititious,
   adventitious, ancillary, annexed, another, appended, appurtenant,
   ascititious, attached, auxiliary, casual, circumstantial,
   collateral, contingent, contributory, excess, extra, extraneous,
   farther, fortuitous, fresh, further, incidental, inessential, more,
   new, nonessential, other, plus, secondary, spare, subordinate,
   subsidiary, superadded, superfluous, supernumerary, supervenient,
   supplemental, supportive, surplus, ulterior, unessential