Most passports allow travellers to go many countries without having to get a visa in advance. Except for entry into the Schengen area where conditions/restrictions are the same to all countries in the zone, all countries have their own rules to accommodate foreigners.
The Henley & Partners agency publishes every year a study on Visa restrictions around the world. The 2013 study shows that the citizens of Northern Europe (Finland , Sweden) and the owners of a British passport have the best passports in the world with the opportunity to travel to 173 countries without a visa (the study considered 219 possible destinations). Citizens of Western European countries, the United States and Canada can travel without a visa in 168 to 172 countries. Asian citizens with the most freedom to travel are the Japanese, the Singaporean, the South Koreans and the Malaysians with 170, 167, 166 and 163 countries respectively accessible without Visa.
For South America, citizens of Argentina have the best passports among their neighboring countries with the ability to travel freely in 147 countries. For the African continent, the citizens of South Africa are freer in their global movement with 94 countries.
The bottom of the ranking is reserved for Iraq (31 countries) and Afghanistan (28 countries).
For new countries, nationals of Kosovo (independence in 2008) can go to 38 countries without a visa and nationals of South Sudan (independence in 2011) can go in 39 countries without a visa (one more than Sudan).
The study ranks countries based on the number of visitable country without a visa, but if you look more closely, what is the passport that allows travel on the most surface as possible? Well, only nationals of Japan and Singapore can go to China, India, Europe and North America without a visa. They are the winners of international travel.