WordNet (r) 3.0 (2006):

chip in
    v 1: contribute to some cause; "I gave at the office" [syn:
         contribute, give, chip in, kick in]

Moby Thesaurus II by Grady Ward, 1.0:

50 Moby Thesaurus words for "chip in":
   acknowledge, be enfranchised, be implicated in, bear a hand,
   bear the expense, break in, chime in, come through, contribute,
   contribute to, cut in, defray, defray expenses, donate to,
   enter into, finance, foot the bill, fund, gift, gift with, give to,
   go Dutch, go into, have suffrage, help decide, honor a bill,
   interrupt, join, join in, kick in, make the scene, partake,
   partake of, participate, participate in, pay for, pay the bill,
   pay the piper, pitch in, pledge, pull an oar, put in, redeem,
   sit in, sit in on, stand the costs, subscribe, sweeten the kitty,
   take part in, vote