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

sock puppet
 n.

    [Usenet: from the act of placing a sock over your hand and talking to it
    and pretending it's talking back] In Usenet parlance, a pseudo through
    which the puppeteer posts follow-ups to their own original message to give
    the appearance that a number of people support the views held in the
    original message. See also astroturfing, tentacle.