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]