The Collaborative International Dictionary of English v.0.48:

Undervalue \Un`der*val"ue\, v. t.
   1. To value, rate, or estimate below the real worth; to
      depreciate.
      [1913 Webster]

   2. To esteem lightly; to treat as of little worth; to hold in
      mean estimation; to despise.
      [1913 Webster]

            In comparison of it I undervalued all ensigns of
            authority.                            --Atterbury.
      [1913 Webster]

            I write not this with the least intention to
            undervalue the other parts of poetry. --Dryden.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Undervalue \Un`der*val"ue\, n.
   A low rate or price; a price less than the real worth;
   undervaluation. --Milton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

undervalue
    v 1: assign too low a value to; "Don't underestimate the value
         of this heirloom-you may sell it at a good price" [syn:
         undervalue, underestimate] [ant: overestimate,
         overvalue]
    2: esteem lightly
    3: lose in value; "The dollar depreciated again" [syn:
       depreciate, undervalue, devaluate, devalue] [ant:
       appreciate, apprise, apprize, revalue]

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "undervalue":
   belittle, decry, deprecate, depreciate, devaluate, devalue,
   disparage, lower, make light of, make little of, make nothing of,
   mark down, minimize, misestimate, misprize, sell short,
   set at naught, set little by, shrug off, think little of,
   think nothing of, underestimate, underprize, underrate,
   underreckon, write down, write off