WordNet (r) 3.0 (2006):

tattletale
    n 1: someone who gossips indiscreetly [syn: tattletale,
         tattler, taleteller, talebearer, telltale,
         blabbermouth]

Moby Thesaurus II by Grady Ward, 1.0:

37 Moby Thesaurus words for "tattletale":
   a tale-bearing animal, betrayer, blab, blabber, blabberer,
   blabbermouth, busybody, delator, fink, gossip, gossip columnist,
   gossiper, gossipmonger, informer, narc, newsmonger, newspaperman,
   peacher, quidnunc, reporter, rumormonger, scandalmonger, snitch,
   snitcher, spy, squealer, stool pigeon, stoolie, tabby, talebearer,
   taleteller, tattler, telltale, tipster, tittle-tattler,
   whistle-blower, yenta