The Collaborative International Dictionary of English v.0.48:

Multitudinous \Mul`ti*tu"di*nous\, a.
   1. Consisting of a multitude; manifold in number or
      condition; as, multitudinous waves. "The multitudinous
      seas." --Shak.
      [1913 Webster]

            A renewed jingling of multitudinous chains. --G.
                                                  Kennan.
      [1913 Webster]

   2. Of or pertaining to a multitude. "The multitudinous
      tongue." --Shak.
      [1913 Webster] -- Mul`ti*tu"di*nous*ly, adv. --
      Mul`ti*tu"di*nous*ness, n.
      [1913 Webster] Multivagant

WordNet (r) 3.0 (2006):

multitudinous
    adj 1: too numerous to be counted; "incalculable riches";
           "countless hours"; "an infinite number of reasons";
           "innumerable difficulties"; "the multitudinous seas";
           "myriad stars"; "untold thousands" [syn: countless,
           infinite, innumerable, innumerous, multitudinous,
           myriad, numberless, uncounted, unnumberable,
           unnumbered, unnumerable]

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "multitudinous":
   abundant, ample, billion, considerable, copious, countless,
   ever so many, full many, generous, heaped-up, innumerable, jillion,
   legion, many, million, much, multifarious, multifold, multiple,
   multitudinal, myriad, no few, not a few, numberless, numerous,
   overflowing, plentiful, populous, quite some, several, sundry,
   superabundant, thousand, uncountable, uncounted, unnumbered,
   untold, various, very many, voluminous, zillion