The Collaborative International Dictionary of English v.0.48:

Moonlighter \Moon"light`er\ (m[=oo]n"l[imac]t`[~e]r), n.
   One who follows an occupation or pastime by moonlight; as:
   (a) A moonshiner.
   (b) In Ireland, one of a band that engaged in agrarian
       outrages by night.
   (c) A serenader by moonlight. [Local, U. S.]
   (d) One who works at a second job in addition to his main
       occupation.
       [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

moonlighter
    n 1: a person who holds a second job (usually after hours)