World Electric Power Guide: http://www.worldstandards.eu/electricity/plugs-and-sockets/
Electric Current Abroad: http://www.ita.doc.gov/media/Publications/pdf/current2002FINAL.pdf (U.S. Department of Commerce)
Precipitated by a colleague looking for the voltage he can expect in Japan. (The answer: 100/200 VAC Single Phase, 200 VAC Delta)
Did you know that 1/2 of Japan is 50 Hz (East) and 1/2 of Japan is 60 Hz (West)? (There is a dividing line running North-South through Shizuoka on Honshu Island.
Two thoughts: if one is traveling to Shizuoka, be ready for anything. And although this seems totally insane, one can imagine that early Japanese products, designed to work for both 50 and 60 Hz in the days before modular power supplies and universal inputs, would have been a lot more competitive on the world market than products designed initially for either 50 Hz or 60Hz, but not both.
Another good resource for world voltages (if you actually want to pay for a small booklet, which I did many moons ago) is British Standards Institute document TH 20338: World Electricity Supplies – Voltages and frequencies (£65.00) which you can purchase online here.