WordNet (r) 3.0 (2006):

chatterbox
    n 1: orchid growing along streams or ponds of western North
         America having leafy stems and 1 greenish-brown and pinkish
         flower in the axil of each upper leaf [syn: stream
         orchid, chatterbox, giant helleborine, Epipactis
         gigantea]
    2: an obnoxious and foolish and loquacious talker [syn:
       chatterer, babbler, prater, chatterbox, magpie,
       spouter]

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "chatterbox":
   agreeable rattle, babbler, big talker, blab, blabber, blabberer,
   blabbermouth, blatherer, busybody, chatterer, conversationalist,
   gabber, gabbler, gasbag, gibble-gabbler, gossip, great talker,
   hot-air artist, idle chatterer, jabberer, jay, magpie,
   moulin a paroles, newsmonger, patterer, prater, prattler, quidnunc,
   rattle, sayer, scandalmonger, speaker, tabby, talker, tattletale,
   utterer, windbag, windjammer, word-slinger