The Collaborative International Dictionary of English v.0.48:

Boundless \Bound"less\, a.
   Without bounds or confines; illimitable; vast; unlimited.
   "The boundless sky." --Bryant. "The boundless ocean."
   --Dryden. "Boundless rapacity." "Boundless prospect of gain."
   --Macaulay.
   [1913 Webster]

   Syn: Unlimited; unconfined; immeasurable; illimitable;
        infinite.
        [1913 Webster] -- Bound"less*ly, adv. --
        Bound"less*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

boundless
    adj 1: seemingly boundless in amount, number, degree, or
           especially extent; "unbounded enthusiasm"; "children with
           boundless energy"; "a limitless supply of money" [syn:
           boundless, unbounded, limitless]

Moby Thesaurus II by Grady Ward, 1.0:

152 Moby Thesaurus words for "boundless":
   Gargantuan, a bit much, abandoned, all-comprehensive,
   all-inclusive, all-knowing, all-powerful, all-seeing, all-wise,
   almighty, amplitudinous, astronomical, awesome, bulky, changeless,
   colossal, cosmic, countless, creating, creative, egregious,
   endless, enormous, eternal, eternally the same, everlasting,
   exaggerated, excessive, exhaustless, exorbitant,
   extending everywhere, extensive, extravagant, extreme, fabulous,
   fancy, galactic, gigantic, glorious, gluttonous, good, hallowed,
   high, highest, holy, huge, hyperbolic, hypertrophied, illimitable,
   illimited, immeasurable, immense, immoderate, immortal, immutable,
   incalculable, incomprehensible, incontinent, indefinite,
   inexhaustible, infinite, infinitely continuous, innumerable,
   inordinate, intemperate, interminable, interminate, just,
   king-size, large, limitless, loving, luminous, majestic, making,
   mammoth, massive, massy, measureless, merciful, monster, monstrous,
   monumental, mountainous, no end of, numinous, omnipotent,
   omnipresent, omniscient, one, out of bounds, out of sight,
   outrageous, outsize, overbig, overdeveloped, overgreat, overgrown,
   overlarge, overmuch, overweening, permanent, perpetual, prodigious,
   radiant, sacred, shaping, shoreless, sizable, sovereign, spacious,
   steep, stiff, stupendous, sumless, supreme, termless, timeless,
   titanic, too much, tremendous, ubiquitous, unbounded, unbridled,
   unchanging, unchecked, uncircumscribed, unconscionable,
   uncontrolled, undefined, undue, unending, unfathomable, universal,
   unlimited, unmeasurable, unmeasured, unnumbered, unplumbed,
   unreasonable, unrestrained, unrestricted, unstoppable, untold,
   vast, voluminous, weighty, without bound, without end,
   without limit, without measure, without number