Courtesy of The Wall Street Journal, a look at the countries they consider frontier markets:

Frontier markets are a varied bunch. The countries that tend to attract the most interest from investors are those that have larger populations, such as Nigeria, Bangladesh and Argentina; those with relatively deep financial markets, such as Kuwait or Romania; or those with abundant natural resources, such as Mongolia, Saudi Arabia and Nigeria. The type of investment opportunities available in each country will depend on the mix of these characteristics, as well as other factors such as GDP growth, governance standards and, of course, location.