The Collaborative International Dictionary of English v.0.48:

womanizer \wom"an*i*zer\, n.
   One who womanizes; a philanderer.
   [PJC]

WordNet (r) 3.0 (2006):

womanizer
    n 1: a man who likes many women and has short sexual
         relationships with them [syn: womanizer, womaniser,
         philanderer]

Moby Thesaurus II by Grady Ward, 1.0:

26 Moby Thesaurus words for "womanizer":
   Casanova, Don Juan, Lothario, chaser, debauchee, gallant,
   gay deceiver, gay dog, lady-killer, libertine, lover-boy, masher,
   philander, philanderer, profligate, rake, rakehell, rip, roue,
   rounder, skirt chaser, swinger, walking phallus, wanton, wolf,
   woman chaser