The Collaborative International Dictionary of English v.0.48:

Gratuity \Gra*tu"i*ty\, n.; pl. Gratuities. [F. gratuit['e],
   or LL. gratuitas.]
   1. Something given freely or without recompense; a free gift;
      a present. --Swift.
      [1913 Webster]

   2. Something voluntarily given in return for a favor or
      service, as a recompense or acknowledgment.
      [1913 Webster]

WordNet (r) 3.0 (2006):

gratuity
    n 1: a relatively small amount of money given for services
         rendered (as by a waiter) [syn: gratuity, tip,
         pourboire, baksheesh, bakshish, bakshis,
         backsheesh]
    2: an award (as for meritorious service) given without claim or
       obligation

Moby Thesaurus II by Grady Ward, 1.0:

63 Moby Thesaurus words for "gratuity":
   Trinkgeld, balance, bonus, boodle, bounty, bribe, bribe money,
   complimentariness, consideration, costlessness, dividend, donative,
   double time, expenselessness, extra, fee, free ride, freebie,
   freeness, gift, gratification, gratuitousness, gravy, grease,
   honorarium, hush money, incentive pay, inducement, labor of love,
   lagniappe, largess, leftover, liberality, margin, no charge,
   overage, overmeasure, overplus, overrun, overset, overstock,
   oversupply, palm, palm oil, payoff, payola, perks, perquisite,
   plus, pourboire, premium, protection, remainder, reward, salve,
   something extra, sop, spare, sportula, surplus, surplusage,
   sweetener, tip