The Collaborative International Dictionary of English v.0.48:

crooner \croon"er\ n.
   a singer of popular ballads.

   Syn: balladeer.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

crooner
    n 1: a singer of popular ballads [syn: crooner, balladeer]

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "crooner":
   Heldentenor, Meistersinger, alto, aria singer, baritenor, baritone,
   bass, basso, basso buffo, basso cantante, basso profundo,
   blues singer, canary, cantatrice, cantor, caroler, chanter,
   chantress, coloratura soprano, comic bass, contralto, countertenor,
   deep bass, diva, dramatic soprano, heroic tenor, hymner,
   improvisator, lead singer, lieder singer, melodist, mezzo-soprano,
   opera singer, prima donna, psalm singer, rock-and-roll singer,
   singer, singstress, songbird, songster, songstress, soprano, tenor,
   torch singer, vocalist, vocalizer, voice, warbler, yodeler