The Collaborative International Dictionary of English v.0.48:

Helpmate \Help"mate`\, n. [A corruption of the "help meet for
   him" of --Genesis ii. 18.--Fitzedward Hall.]
   A helper; a companion; specifically, a wife.
   [1913 Webster]

         In Minorca the ass and the hog are common helpmates,
         and are yoked together in order to turn up the land.
                                                  --Pennant.
   [1913 Webster]

         A waiting woman was generally considered as the most
         suitable helpmate for a parson.          --Macaulay.
   [1913 Webster]

WordNet (r) 3.0 (2006):

helpmate
    n 1: a helpful partner [syn: helpmate, helpmeet]

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "helpmate":
   acolyte, adjutant, agent, aid, aide, aide-de-camp, aider,
   assistant, attendant, auxiliary, best man, better half, coadjutant,
   coadjutor, coadjutress, coadjutrix, common-law wife, concubine,
   deputy, executive officer, feme, feme covert, goodwife, goody,
   help, helper, helpmeet, lady, lieutenant, married woman, matron,
   old lady, old woman, paranymph, paraprofessional, rib, second,
   servant, sideman, squaw, supporting actor,
   supporting instrumentalist, wedded wife, wife, woman



The Devil's Dictionary (1881-1906):

HELPMATE, n.  A wife, or bitter half.

    "Now, why is yer wife called a helpmate, Pat?"
        Says the priest.  "Since the time 'o yer wooin'
    She's niver [sic] assisted in what ye were at --
        For it's naught ye are ever doin'."

    "That's true of yer Riverence [sic]," Patrick replies,
        And no sign of contrition envices;
    "But, bedad, it's a fact which the word implies,
        For she helps to mate the expinses [sic]!"
                                                         Marley Wottel