WordNet (r) 3.0 (2006):

zillion
    adj 1: very large indeterminate number; "a zillion people were
           there"
    n 1: a very large indefinite number (usually hyperbole); "there
         were millions of flies" [syn: million, billion,
         trillion, zillion, jillion, gazillion]

Moby Thesaurus II by Grady Ward, 1.0:

52 Moby Thesaurus words for "zillion":
   a billion, a crore, a lakh, a million, a myriad, a nonillion,
   a quadrillion, a thousand, a zillion, astronomical number, billion,
   considerable, decillion, duodecillion, ever so many, full many,
   googol, googolplex, heaped-up, infinitude, infinity, jillion,
   large number, many, million, multifarious, multifold, multiple,
   multitudinal, multitudinous, myriad, no few, nonillion, not a few,
   novemdecillion, numerous, octillion, octodecillion, quadrillion,
   quattuordecillion, quindecillion, quintillion, quite some,
   septillion, sexdecillion, sextillion, thousand, tredecillion,
   trillion, undecillion, very many, vigintillion