The Jargon File (version 4.4.7, 29 Dec 2003):

address harvester
 n.

    A robot that searches web pages and/or filters netnews traffic looking for
    valid email addresses. Some address harvesters are benign, used only for
    compiling address directories. Most, unfortunately, are run by miscreants
    compiling address lists to spam. Address harvesters can be foiled by a 
    teergrube.