The Collaborative International Dictionary of English v.0.48:

Yes \Yes\, adv. [OE. yis, [yogh]is, [yogh]es, [yogh]ise, AS.
   gese, gise; probably fr. ge['a] yea + sw[=a] so. [root]188.
   See Yea, and So.]
   Ay; yea; -- a word which expresses affirmation or consent; --
   opposed to no.
   [1913 Webster]

   Note: Yes is used, like yea, to enforce, by repetition or
         addition, something which precedes; as, you have done
         all this -- yes, you have done more. "Yes, you despise
         the man books confined." --Pope.
         [1913 Webster]

   Note: "The fine distinction between `yea' and `yes,' `nay'
         and `no,' that once existed in English, has quite
         disappeared. `Yea' and `nay' in Wyclif's time, and a
         good deal later, were the answers to questions framed
         in the affirmative. `Will he come?' To this it would
         have been replied, `Yea' or `Nay', as the case might
         be. But, `Will he not come?' To this the answer would
         have been `Yes' or `No.' Sir Thomas More finds fault
         with Tyndale, that in his translation of the Bible he
         had not observed this distinction, which was evidently
         therefore going out even then, that is, in the reign of
         Henry VIII.; and shortly after it was quite forgotten."
         --Trench.
         [1913 Webster]

WordNet (r) 3.0 (2006):

yes
    n 1: an affirmative; "I was hoping for a yes" [ant: no]

Moby Thesaurus II by Grady Ward, 1.0:

150 Moby Thesaurus words for "yes":
   Australian ballot, Hare system, OK, Roger, abide by, absolutely,
   accede, accept, acclaim, accordantly, acquiesce, acquiesce in,
   acquiescently, affirmative, affirmative attitude, affirmatively,
   affirmativeness, agree, agree to, agree with, agreeably, agreed,
   all right, alright, alrighty, amen, applaud, approvingly,
   as you say, assent, assentingly, assuredly, aye, ballot,
   beyond a doubt, buy, by all means, canvass, canvassing,
   casting vote, certainly, cheer, compliantly, comply, consent,
   consentingly, counting heads, cumulative voting, da, deciding vote,
   division, enfranchisement, exactly, fagot vote, favorably, fine,
   franchise, give the nod, gladly, good, good enough, graveyard vote,
   hail, hand vote, hear, hold with, in toto, indeed, indeedy, ja,
   just so, list system, mais oui, most assuredly, naturally,
   naturellement, nay, no, nod, nod assent, nod of assent,
   nontransferable vote, of course, okay, oui, plebiscite,
   plebiscitum, plumper, plural vote, poll, polling, positively,
   precisely, preferential voting, proportional representation, proxy,
   quite, rather, really, receive, record vote, referendum,
   representation, right, right to vote, righto, rising vote, say,
   secret ballot, show of hands, single vote, snap vote, straw vote,
   subscribe, subscribe to, suffrage, sure, sure thing, surely,
   take kindly to, thumbs-up, to be sure, transferable vote, truly,
   undoubtedly, unquestionably, very well, viva voce, voice,
   voice vote, vote, vote for, voting, voting right, welcome,
   well and good, why yes, willingly, write-in, write-in vote, yea,
   yea-saying, yeah, yeas and nays, yep, yes indeed, yes indeedy,
   yes sir, yes sirree, yield assent