WordNet (r) 3.0 (2006):

kitchen cabinet
    n 1: an inner circle of unofficial advisors to the head of a
         government [syn: kitchen cabinet, brain trust]

Moby Thesaurus II by Grady Ward, 1.0:

42 Moby Thesaurus words for "kitchen cabinet":
   British Cabinet, Sanhedrin, US Cabinet, advisory body,
   advisory council, assembly, association, bench, board,
   body of advisers, borough council, brain trust, cabinet, camarilla,
   chamber, city council, common council, conference, congress,
   consultative assembly, council, council fire, council of ministers,
   council of state, council of war, county council, court,
   deliberative assembly, diet, directory, divan, junta, legislature,
   ministry, parish council, privy council, shadow cabinet, soviet,
   staff, syndicate, synod, tribunal