A canonical list of the 20 biggest megacorps in Japan, as of 2002, with 2001 revenue figures (data stolen from the Fortune Global 500 list):
  1. Toyota, $120.8bn
  2. Mitsubishi, $105.8bn
  3. Mitsui, $101.2bn
  4. NTT, $93.4bn
  5. Itochu, $91.2bn
  6. Sumitomo, $77.1bn
  7. Marubeni, $71.7bn
  8. Hitachi, $63.9bn
  9. Nippon Seimei, $63.8bn
  10. Sony, $60.6bn
  11. Honda, $58.9bn
  12. Matsushita (National/Panasonic), $55.0bn
  13. Nissan, $49.6bn
  14. Nissho Iwai, $43.7bn
  15. Dai-Ichi Kangyo, $43.1bn
  16. Toshiba, $43.1bn
  17. Tokyo Electric, $41.8bn
  18. Mizuho, $41.4bn
  19. NEC, $40.8bn (part of Sumitomo)
  20. Fujitsu, $40.0bn
In comparison:

Gross national product of Norway: $124.1bn
Estimated net worth of Bill Gates: $29.8bn
Price of a stealth bomber: $2bn

National debt of the United States: $6.2 trillion

Other companies that are not on this list, but are quite massive and scary in their own right, include Nomura, Nippon Steel, JR, Tokyo Marine, Sharp, NKK, Yasuda, and Daiwa.