WordNet (r) 3.0 (2006):

fink
    n 1: someone acting as an informer or decoy for the police [syn:
         fink, snitch, snitcher, stoolpigeon, stool
         pigeon, stoolie, sneak, sneaker, canary]
    v 1: take the place of work of someone on strike [syn: fink,
         scab, rat, blackleg]
    2: confess to a punishable or reprehensible deed, usually under
       pressure [syn: confess, squeal, fink]

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "fink":
   bear witness against, betray, betrayer, blab, blabber, blabberer,
   blabbermouth, blackleg, blow the whistle, canary, delator,
   inform against, inform on, informer, narc, nark, peach, peacher,
   rat, scab, sell out, snitch, snitch on, snitcher, spy, squeal,
   squealer, stool, stool pigeon, stoolie, strikebreaker, talebearer,
   tattle, tattler, tattletale, tell on, telltale, testify against,
   tipster, turn informer, whistle-blower