The Collaborative International Dictionary of English v.0.48:

Cooperate \Co*["o]p"er*ate\, v. i. [imp. & p. p. Cooperated;
   p. pr. & vb. n. Cooperating.] [L. cooperatus, p. p. of
   cooperari to cooperate; co + operari to work, opus work. See
   Operate.]
   To act or operate jointly with another or others; to concur
   in action, effort, or effect.
   [1913 Webster]

         Whate'er cooperates to the common mirth. --Crashaw.
   [1913 Webster]

WordNet (r) 3.0 (2006):

cooperate
    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]

Moby Thesaurus II by Grady Ward, 1.0:

167 Moby Thesaurus words for "cooperate":
   accord, acquiesce, act in concert, act together, affiliate, agree,
   ally, alternate, amalgamate, answer to, apportion, assent, assist,
   associate, assort with, band, band together, bandy,
   be agreeable to, be consistent, be dying to, be eager, be game,
   be in cahoots, be in league, be of one, be open to, be partners in,
   be quits with, be ready, be spoiling for, be uniform with,
   be willing, change, check, chime, club together, coact, coadunate,
   coalesce, cohere, coincide, collaborate, collude, combine, commute,
   compensate, concert, concord, concur, confederate, conform,
   conform with, conjoin, connive, consent, consist with, consolidate,
   conspire, contribute, correspond, counterchange, divide with,
   divvy up with, do business with, dovetail, engage, exchange,
   fall in together, federate, fit together, fuse, get back at,
   get even with, get heads together, get together, give and take,
   go along with, go even stephen, go fifty-fifty, go halvers,
   go halves, go partners, go shares, go together, go with, halve,
   hang together, happen together, harmonize, help, hit,
   hold together, hook up, incline, interact, interchange, interlace,
   interlock, intermesh, interplay, intersect, intertwine, interweave,
   interwork, jibe, join, join forces, join in, join together,
   keep together, league, league together, lean, lend a hand, lock,
   logroll, look kindly upon, make common cause, match, merge, mesh,
   mortise, not hesitate to, overlap, parallel, participate, partner,
   pay back, permute, play ball, plunge into, pull together,
   put heads together, reciprocate, register, register with, requite,
   respond, respond to, retaliate, return, return the compliment,
   share, share in, sing in chorus, sort with, split the difference,
   square, square with, stand together, swap, switch, synchronize,
   synergize, tally, team up, throw in together, tie in, tie up,
   trade, transpose, unite, unite efforts, work together,
   would as leave, would as lief