The Collaborative International Dictionary of English v.0.48:

Lagniappe \La*gniappe\, Lagnappe \La*gnappe"\, n. [Also spelled
   lagnappe.][Etym. uncertain.]
   1. In Louisiana, a trifling present given to customers by
      tradesmen; a gratuity.
      [1913 Webster]

            Lagniappe . . .is something thrown in, gratis, for
            good measure.                         --Mark Twain.
      [Webster 1913 Suppl.]

   2. A tip or gratuity.
      [PJC]

   3. Hence: Anything obtained gratuitously or unexpectedly.
      [PJC]

WordNet (r) 3.0 (2006):

lagniappe
    n 1: a small gift (especially one given by a merchant to a
         customer who makes a purchase)

Moby Thesaurus II by Grady Ward, 1.0:

64 Moby Thesaurus words for "lagniappe":
   Trinkgeld, balance, bonus, bonus system, bounty, bribe,
   consideration, decoration, dividend, donative, double time, extra,
   extra added attraction, extra dash, fee, filigree, filling, fillip,
   flourish, frill, fringe benefit, gratuity, gravy, grease,
   honorarium, incentive pay, inducement, largess, leftover,
   liberality, margin, ornament, overage, overmeasure, overplus,
   overrun, overset, overstock, oversupply, overtime pay, padding,
   palm, palm oil, perks, perquisite, perquisites, plus, pourboire,
   premium, remainder, salve, solatium, something extra, spare,
   sportula, stuffing, superaddition, surplus, surplusage, sweetener,
   tip, trimming, twist, wrinkle