The Collaborative International Dictionary of English v.0.48:

Endless \End"less\, a. [AS. endele['a]s. See End.]
   1. Without end; having no end or conclusion; perpetual;
      interminable; -- applied to length, and to duration; as,
      an endless line; endless time; endless bliss; endless
      praise; endless clamor.
      [1913 Webster]

   2. Infinite; excessive; unlimited. --Shak.
      [1913 Webster]

   3. Without profitable end; fruitless; unsatisfying. [R.] "All
      loves are endless." --Beau. & Fl.
      [1913 Webster]

   4. Void of design; objectless; as, an endless pursuit.
      [1913 Webster]

   5. having a linear or ribbonlike form with the two ends
      connected together, such as in a circle, ellipse, torus,
      or any other closed loop. Belts or chains used in drive
      mechanisms are considered endless in this sense. See
      endless chain.
      [PJC]

   Endless chain, a chain which is made continuous by uniting
      its two ends.

   Endless screw. (Mech.) See under Screw.

   Syn: Eternal; everlasting; interminable; infinite; unlimited;
        incessant; perpetual; uninterrupted; continual;
        unceasing; unending; boundless; undying; imperishable.
        [1913 Webster]

WordNet (r) 3.0 (2006):

endless
    adj 1: tiresomely long; seemingly without end; "endless
           debates"; "an endless conversation"; "the wait seemed
           eternal"; "eternal quarreling"; "an interminable sermon"
           [syn: endless, eternal, interminable]
    2: infinitely great in number; "endless waves"
    3: having no known beginning and presumably no end; "the
       dateless rise and fall of the tides"; "time is endless";
       "sempiternal truth" [syn: dateless, endless,
       sempiternal]
    4: having the ends united so as to form a continuous whole; "an
       endless chain"

Moby Thesaurus II by Grady Ward, 1.0:

133 Moby Thesaurus words for "endless":
   ageless, all-comprehensive, all-inclusive, amaranthine,
   articulated, boundless, catenated, ceaseless, coeternal,
   concatenated, connected, constant, continual, continued,
   continuing, continuous, countless, cyclical, dateless,
   de longue haleine, deathless, direct, eternal, eterne, ever-being,
   ever-durable, ever-during, everlasting, everliving, exhaustless,
   extended, extending everywhere, featureless, filled out, gapless,
   illimitable, illimited, immeasurable, immediate, immemorial,
   immense, immortal, incalculable, incessant, incomprehensible,
   indefinite, indestructible, inexhaustible, infinite,
   infinitely continuous, innumerable, interminable, interminate,
   joined, jointless, lengthy, limitless, linked, long,
   long-drawn-out, long-spun, long-winded, longiloquent, measureless,
   monotonous, never-ceasing, never-ending, no end of, nonstop,
   nonterminating, nonterminous, numberless, olamic, overlong, padded,
   perdurable, perennial, periodic, permanent, perpetual, prolix,
   protracted, recurrent, repetitive, round-the-clock, running,
   seamless, sempiternal, serried, shoreless, smooth, spun-out,
   stable, steady, straight, sumless, talkative, termless, timeless,
   twenty-four-hour, unbounded, unbroken, unceasing, uncircumscribed,
   uncounted, undifferentiated, undying, unending, unfathomable,
   uniform, unintermitted, unintermittent, unintermitting,
   uninterrupted, universal, unlimited, unmeasurable, unmeasured,
   unnumbered, unplumbed, unrelenting, unrelieved, unremitting,
   unstopped, untold, verbose, windy, without bound, without end,
   without limit, without measure, without number, wordy