WordNet (r) 3.0 (2006):

write in
    v 1: cast a vote by inserting a name that does not appear on the
         ballot; "Many voters wrote in the names of strangers"
    2: write to an organization; "Many listeners wrote in after the
       controversial radio program"

Moby Thesaurus II by Grady Ward, 1.0:

52 Moby Thesaurus words for "write in":
   Australian ballot, Hare system, aye, ballot, canvass, canvassing,
   casting vote, counting heads, cumulative voting, deciding vote,
   division, enfranchisement, fagot vote, franchise, graveyard vote,
   hand vote, list system, nay, no, nontransferable vote, plebiscite,
   plebiscitum, plumper, plural vote, poll, polling,
   preferential voting, proportional representation, proxy,
   record vote, referendum, representation, right to vote,
   rising vote, say, secret ballot, show of hands, single vote,
   snap vote, straw vote, suffrage, transferable vote, viva voce,
   voice, voice vote, vote, voting, voting right, write-in vote, yea,
   yeas and nays, yes