WordNet (r) 3.0 (2006):

counterweight
    n 1: a weight that balances another weight [syn:
         counterweight, counterbalance, counterpoise,
         balance, equalizer, equaliser]
    v 1: constitute a counterweight or counterbalance to [syn:
         counterweight, counterpoise, counterpose]

Moby Thesaurus II by Grady Ward, 1.0:

72 Moby Thesaurus words for "counterweight":
   act drop, asbestos, asbestos board, backdrop, balance,
   balance rudder, balance wheel, ballast, batten, border, cloth,
   consideration, coulisse, counterbalance, counterblast,
   countercurrent, counterforce, counterinfluence, counterpoise,
   counterpressure, crosscurrent, curtain, curtain board, cyclorama,
   decor, drop, drop curtain, equipoise, equivalent, fin,
   fire curtain, fixative, flat, flipper, flywheel, foul wind,
   give-and-take, governor, gyroscope, hairspring, hanging, head wind,
   keel, makeweight, mordant, offset, pendulum, quid pro quo, rag,
   scene, scenery, screen, setoff, shock absorber, side scene,
   something of value, springs, stabilizator, stage screw, stiffening,
   tab, tableau, tail plane, teaser, tit for tat, tormentor,
   transformation, transformation scene, undercurrent, wing, wingcut,
   woodcut