NOTE: Type C plugs can also be used with E and F  and some L receptacles.  
This type of plug is very common in most "universal" adaptor sets.
(Though the diagram shows a circular plug, quite often Type C plugs have a rectangular form factor, as in the photo above.)

Where found:

Albania, Algeria, Angola, Argentina, Austria, Azores, Balearic Islands, Bangladesh, Belgium, Bolivia, Bosnia, Brazil, Bulgaria, Burkina Faso, Burundi, Cameroon, Canary Islands, Cape Verde, Central African Republic, Channel Islands, Chile, Comoros, Congo, Croatia, Dem. Rep. of Congo (Zaire), Cote d'Ivoire (Ivory Coast), Cyrpus, Denmark, Djibouti, Egypt, El Salvador, Equatorial Guinea, Eritrea, Faeroe Islands, Finland, French Guiana, Gabon, Germany, Gibraltar, Greece, Greenland, Guadeloupe, Guinea, Guinea-Bissau, Hungary, Iceland, India, Indonesia, Iran, Iraq, Isle of Man, Israel, Italy, Kazakhstan, Korea, Kuwait, Laos, Lebanon, Lithuania, Luxembourg, Macedonia, Madagascar, Madeira, Mali, Martinique, Mauritania, Mauritius, Monaco, Morocco, Mozambique, Myanmar, Nepal, Netherlands, Niger, Norway, Oman, Pakistan, Paraguay, Peru, Philippines, Poland, Portugal, Romania, Russia, Rwanda, St. Vincent, Senegal, Serbia, Slovenia, Somalia, Spain, Sudan, Suriname, Sweden, Switzerland, Syria, Thailand, Togo, Tunisia, Turkey, Uruguay, Vietnam, Yugoslavia (former),  Zambia.