The Collaborative International Dictionary of English v.0.48:

blabbermouth \blabbermouth\ n.
   someone who gossips indiscreetly.

   Syn: blabber, tattletale, taleteller, talebearer, telltale.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "blabbermouth":
   babbler, betrayer, blab, blabber, blabberer, chatterer, delator,
   fink, gabber, gossip, informer, jabberer, magpie, narc, peacher,
   prater, prattler, snitch, snitcher, spy, squealer, stool pigeon,
   stoolie, talebearer, tattler, tattletale, telltale,
   whistle-blower