Coal.
That depends a lot on what you mean by "this source".
Figures for 2006 showed coal at 49 percent, and both nuclear and natural gas at about 19 percent.
it is one of the leading ones
Coal
Oil
The burning of fossil fuels (coal, oil and natural gas) in industry, transport and the generation of electricity, which releases carbon dioxide (CO2).
Coal is the most used source for electricity in the US. World wide, other countries, I don't know.
Coal - about 50 percent of total
Coal is still primarily used in the US to generate power (~47% of all electricity generated), however natural gas is being used increasingly to generate electricity (~20% of all electrical generation).
Fossil fuels, suck as burning coal or using oil
As of 2013, the amount of the US's electricity that comes from coal is approximately 42% This makes up the mass majority of electricity service in the country. Natural gas is the second most popular source.
Electricity supplies in the US contain about 19 percent nuclear generation, so anything which uses electricity from the mains uses nuclear power