Answer: Donner and Blitzen
Santa Claus's sleigh is meant to be led by eight reindeer: Dasher, Dancer, Prancer, Vixen, Comet, Cupid, Donner (also Donder or Dunder) and Blixem (also Blixen or Blitzen). Sometimes Rudolph is added too.
Donner and Blitzen come from the Dutch words for <em>thunder</em> and <em>lightning</em> (in modern Dutch, "donder" and "bliksem"). They are also similar to the German equivalent of these words ("donner" and "blixem").