The Pew Forum has released a new study: A Report on the Size and Distribution of the World's Major Religious Groups as of 2010.
Of the world's 6.9 billion inhabitants: Christians make up 31.5%, Muslims 23.2%, unaffiliated 16.3%, Hindus 15.0%, Buddhists 7.1%, folk and traditional religionists 5.9%, and other 0.8%.
The unaffiliated, making up 1 in 6 of the world's population, are an interesting category. The large majority of them are in the Asia-Pacific region, with many of those in religiously-stifled China. Some of those unaffiliated actually believe in God or other deities. Unaffiliated make up the majority in China, Czech Republic, Estonia, Hong Kong, Japan, and North Korea. Unfortunately, some outlets are reporting "Unbelief" as the third largest religion worldwide (e.g., here), but unaffiliated does not equal unbelief.