There are two methods of GDP calculation: nominal GDP attempts to compare countries using current exchange rates to give an assessment of their clout within the global market. This naturally biases countries with stronger currencies.
Purchasing Power Parity or PPP GDP, on the other hand, tries to take into account that one dollar can buy more in some countries and less in others. It is a better gauge of the internal size of each market.
In the nominal GDP method, we can see that the developed world leads the pack, but that China has already broken into this exclusive club, and is now the second largest economy in the world by both measures.
When we look at PPP GDP, all of the BRIC countries (China, India, Brazil and Russia) are all within the top 10.
Here is the Top 10, as listed by PPP GDP, using 2010 GDP figures: