Not only can the socket shape be different, but different countries use different voltages and wattage, which means some appliances could fail to work in one country's outlets and cause a fire when plugged into another's.
Regarding shape of sockets, Japanese plug sockets accept plugs from the following countries (and vise versa):
American Samoa, Anguilla, Antigua, Aruba, Bahamas, Bangladesh, Barbados, Bermuda, Bolivia, Brazil, Cambodia, Canada, Cayman Islands, China, Colombia, Costa Rica, Cuba, Ecuador, El Salvador, Guam, Guatemala, Guyana, Haiti, Honduras, Jamaica, Japan, Laos, Lebanon, Liberia, Maldives, Mexico, Micronesia, Montserrat, Netherlands Antilles, Nicaragua, Niger, Okinawa, Panama, Peru, Philippines, Puerto Rico, St. Vincent, Saudi Arabia, Tahiti, Taiwan, Thailand, United States, Venezuela, Vietnam, Virgin Islands (U.S.& British), Yemen.
|Beat the three-pronged system|
with a 100 yen adapter!
...just be sure it's safe.
However, you must make sure that you use the correct transformer for vastly different voltage/wattages. Also note that if you use an item at a higher than rated voltage, you can no longer claim the product's liability for any fault or incident.
Usually, for electronics from/to the US, there shouldn't be a problem not using a transformer, but be especailly careful with things involving heat: Hairdryers, straighteners, and stuff like that. Running a Japanese one of these back in the States will result in a lot more heat being generated; this will either blow a fuse or be a fire risk. Consult the manual or item itself for a rundown of the acceptable voltage and wattage.
American products used in Japan will be slightly (but reportedly not significantly) cooler. (From Steve: I use a travel-sized hairdryer that I bought in the States. The fan is a little less peppy than in its original country, but I have used in Japan without any problems for more than 3 years.)
For the UK, remember that bringing heat-related items to Japan is almost pointless - they do crazy unpredictable things. They are not really more expensive to buy here, and you'll just have to bite the bullet on this one.
Usually, laptop chargers, phone chargers, camera chargers, etc., are designed to work at many different wattages. Check the rear of your product or manual for the range. The same often applies to TVs, etc.
Here's a comparison of Japan vs. the major ex-pat exporting states (taken from this extensive list), also including plug types (ABC etc):
|Japan||100 V||50/60 Hz*||A & B||*Eastern Japan 50 Hz (Tokyo, Kawasaki, Sapporo, Yokohoma, and Sendai); Western Japan 60 Hz (Osaka, Kyoto, Nagoya, Hiroshima)|
|USA||120V||60 Hz||A & B|
|UK||230 V||50 Hz||G|
|Australia||240 V||50 Hz||I|
|New Zealand||230 V||50 Hz||I|
|Brazil||110/220 V*||60 Hz||A & B, C||*127 V found in states of Bahia, ParanÃ¡ (including Curitiba), Rio de Janeiro, SÃo Paulo and Minas Gerais (though 220 V may be found in some hotels). Other areas are 220 V only, with the exception of Fortaleza (240 V). Outlets (click for more) are often a combination of type A and C and can accept either type plug.|
|Philippines||220 V||60 Hz||A, B, C|
Although Japan and the US look very similar when compared to some of the others, I'll say again to be careful with items involving heat. A Japanese product designed to work at 100V will be 44% hotter at 120V. However, I have heard that more basic items such as rice cookers and denki pots will usually be fine.