The Collaborative International Dictionary of English v.0.48:

collaborate \col*lab"o*rate\, v. i.
   1. to work together with another toward a common goal,
      especially in an intellectual endeavor; as, four chemists
      collaborated on the synthesis of the compound; three
      authors collaborated in writing the book.
      [PJC]

   2. to willingly cooperate with an enemy, especially an enemy
      nation occupying one's own country.
      [PJC]

WordNet (r) 3.0 (2006):

collaborate
    v 1: work together on a common enterprise of project; "The
         soprano and the pianist did not get together very well";
         "We joined forces with another research group" [syn:
         collaborate, join forces, cooperate, get together]
    2: cooperate as a traitor; "he collaborated with the Nazis when
       they occupied Paris"

Moby Thesaurus II by Grady Ward, 1.0:

105 Moby Thesaurus words for "collaborate":
   accord, acquiesce, act in concert, act the traitor, act together,
   affiliate, agree, ally, amalgamate, associate, author, band,
   band together, be agreeable to, be dying to, be eager, be game,
   be in cahoots, be in league, be open to, be ready, be spoiling for,
   be willing, club together, coact, coadunate, coalesce, coauthor,
   coincide, collude, combine, compose, concert, concord, concur,
   confederate, conjoin, connive, consent, consolidate, conspire,
   cooperate, correspond, dash off, do business with, editorialize,
   federate, formulate, fraternize, free-lance, fuse,
   get heads together, get together, ghost, ghostwrite, go along with,
   go partners, hang together, happen together, harmonize,
   hold together, hook up, incline, indite, join, join forces,
   join in, join together, keep together, knock off, knock out,
   league, league together, lean, look kindly upon, make common cause,
   merge, not hesitate to, novelize, pamphleteer, partner, play ball,
   plunge into, prepare, produce, pull together, put heads together,
   quisle, reciprocate, scenarize, stand together, synchronize,
   synergize, team up, throw in together, throw on paper, tie in,
   tie up, turn against, unite, unite efforts, work together,
   would as leave, would as lief, write