The Collaborative International Dictionary of English v.0.48:

Countless \Count"less\ (-l?s), a.
   Incapable of being counted; not ascertainable; innumerable.
   [1913 Webster]

WordNet (r) 3.0 (2006):

countless
    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:

55 Moby Thesaurus words for "countless":
   abundant, all-comprehensive, all-inclusive, ample, boundless,
   copious, endless, eternal, exhaustless, extending everywhere,
   generous, illimitable, illimited, immeasurable, immense,
   incalculable, incomprehensible, inexhaustible, infinite,
   infinitely continuous, innumerable, interminable, interminate,
   limitless, many, measureless, much, multitudinous, no end of,
   numberless, numerous, overflowing, perpetual, plentiful, shoreless,
   sumless, superabundant, termless, unbounded, uncircumscribed,
   uncountable, uncounted, unfathomable, universal, unlimited,
   unmeasurable, unmeasured, unnumbered, unplumbed, untold,
   without bound, without end, without limit, without measure,
   without number