The Collaborative International Dictionary of English v.0.48:

Inestimable \In*es"ti*ma*ble\, a. [L. inaestimabilis: cf. F.
   inestimable. See In- not, and Estimate.]
   Incapable of being estimated or computed; especially, too
   valuable or excellent to be measured or fully appreciated;
   above all price; as, inestimable rights or privileges.
   [1913 Webster]

         But above all, for thine inestimable love. --Bk. of
                                                  Com. Prayer.
   [1913 Webster]

         Science is too inestimable for expression by a money
         standard.                                --Lyon
                                                  Playfair.

   Syn: Incalculable; invaluable; priceless.
        [1913 Webster]

WordNet (r) 3.0 (2006):

inestimable
    adj 1: beyond calculation or measure; "of incalculable value";
           "an incomputable amount"; "jewels of inestimable value";
           "immeasurable wealth" [syn: incomputable,
           inestimable, immeasurable]

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "inestimable":
   beyond price, certain, dear, golden, good as gold, immeasurable,
   ineluctable, ineludible, inescapable, invaluable, measureless,
   necessary, of great price, precious, priceless, rich, unavoidable,
   uncountable, unevadable, unmeasurable, unmeasured, valuable,
   worthy