The Collaborative International Dictionary of English v.0.48:

Sweetener \Sweet"en*er\, n.
   One who, or that which, sweetens; one who palliates; that
   which moderates acrimony.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sweetener
    n 1: something added to foods to make them taste sweeter [syn:
         sweetening, sweetener]
    2: anything that serves as an enticement [syn: bait, come-
       on, hook, lure, sweetener]

Moby Thesaurus II by Grady Ward, 1.0:

73 Moby Thesaurus words for "sweetener":
   Trinkgeld, ambrosia, artificial sweetener, bait, blackstrap, bonus,
   bounty, bribe, calcium cyclamate, cane syrup, carrot, clover honey,
   comb honey, consideration, corn syrup, cyclamates, donative,
   double time, edulcoration, encouragement, fee, fillip, gratuity,
   gravy, grease, honey, honeycomb, honeydew, honeypot, honorarium,
   incentive, incentive pay, incitement, inducement, interest,
   invitation, lagniappe, largess, liberality, lure, maple syrup,
   molasses, nectar, palm oil, payment, percentage, perks, perquisite,
   persuasive, pourboire, premium, profit, provocation, reward,
   saccharification, saccharin, salve, sodium cyclamate,
   something extra, sorghum, sportula, stimulation, stimulative,
   stimulus, sugar, sugar-making, sugaring off, sweetening, sweets,
   syrup, tip, treacle, whet