The Collaborative International Dictionary of English v.0.48:

Contributory \Con*trib"u*to*ry\, a.
   Contributing to the same stock or purpose; promoting the same
   end; bringing assistance to some joint design, or increase to
   some common stock; contributive. --Milton.
   [1913 Webster]

         Bonfires of contributory wood.           --Chapman.
   [1913 Webster]

   Contributory negligence (Law), negligence by an injured
      party, which combines with the negligence of the injurer
      in producing the injury, and which bars recovery when it
      is the proximate cause of the injury. --Wharton.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Contributory \Con*trib"u*to*ry\, n.; pl. Contributories.
   One who contributes, or is liable to be called upon to
   contribute, as toward the discharge of a common indebtedness.
   --Abbott.
   [1913 Webster]

WordNet (r) 3.0 (2006):

contributory
    adj 1: tending to bring about; being partly responsible for;
           "working conditions are not conducive to productivity";
           "the seaport was a contributing factor in the growth of
           the city"; "a contributory factor" [syn: conducive,
           contributing(a), contributive, contributory,
           tributary]

Moby Thesaurus II by Grady Ward, 1.0:

43 Moby Thesaurus words for "contributory":
   accessory, additional, adjuvant, aidful, ancillary, another,
   appurtenant, auxiliary, beneficial, collateral, concessive,
   conducive, constructive, donative, extra, farther, fresh, further,
   furthersome, good for, helpful, intestate, more, new, other, plus,
   positive, profitable, remedial, salutary, serviceable, spare,
   subservient, subsidiary, supernumerary, supplemental,
   supplementary, surplus, testamentary, testate, therapeutic,
   ulterior, useful