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]