The UN publishes an annual report on human development each year, which produces a composite score for almost all nations in the world, based on indicators like per capita income, life expectancy, literacy, higher education, diffusion of technology, patents generated, etc.
The world map below displays the approximate development level of the world's nations, with the greener nations being more developed, and the redder nations being less developed (source : Wikipedia).
Wealth and poverty can be measured in both relative and absolute terms. In absolute terms, the progress in human development that has been made since 1975 is stunning. See the table of Human Development Index progress here.
In 1975, only about 20 nations had acheived a score of 0.800 or greater (the level that corresponds to being a 'developed' country). By 2003, approximately 57 countries have achieved 'developed' status.
This can be linked entirely to the diffusion of technology, spread of globalization, and the emergence of the United States as the dominant economic model in the world, creating win-win trade arrangements with many other countries.