The Collaborative International Dictionary of English v.0.48:

Tagger \Tag"ger\, n.
   1. One who, or that which, appends or joins one thing to
      another.
      [1913 Webster]

   2. That which is pointed like a tag.
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            Hedgehogs' or procupines' small taggers. --Cotton.
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   3. pl. Sheets of tin or other plate which run below the
      gauge. --Knight.
      [1913 Webster]

   4. A device for removing taglocks from sheep. --Knight.
      [1913 Webster]
      [1913 Webster]

WordNet (r) 3.0 (2006):

tagger
    n 1: someone who assigns labels to the grammatical constituents
         of textual matter
    2: someone who appends or joins one thing to another; "a theory
       that was simply added on by some anonymous tagger"
    3: a computer program that attaches labels to the grammatical
       constituents of textual matter [syn: tagger, tagging
       program]