http://www.taxpolicycenter.org/briefing-book/background/numbers/revenue.cfm

The Numbers: What are the federal government’s sources of revenue?

Individual income taxes and payroll taxes now account for four out of every five federal revenue dollars. Corporate income taxes contribute another 12 percent. Excise taxes, estate and gift taxes, customs duties, and miscellaneous receipts (earnings of the Federal Reserve System and various fees and charges) make up the balance. The composition of tax revenue has changed markedly over the past half century, with payroll taxes contributing an increasing, and corporate income and excise taxes a decreasing, share of the total, but the share provided by individual income taxes has remained roughly constant.

• In 2008 the federal government collected $2.5 trillion, an amount equal to 17.7 percent of GDP. Federal revenue has ranged from 14.4 to 20.9 percent of GDP over the past five decades, averaging 18.2 percent.
• The individual income tax has been the largest single source of federal revenue since 1950, averaging just over 8 percent of GDP.
• Payroll taxes swelled following the creation of Medicare in 1965. Taxes for Medicare, combined with periodic increases in Social Security taxes, caused payroll tax revenue to grow from 1.6 percent of GDP in 1950 to more than 6 percent since 1990. Payroll taxes also include railroad retirement, unemployment insurance, and federal workers’ pension contributions.
• Revenue from the corporate income tax fell from between 5 and 6 percent of GDP in the early 1950s to 2.1 percent of GDP in 2008.
• Excise taxes fell steadily throughout the same period, from nearly 3 percent of GDP in 1950 to 0.5 percent in recent years.
• The remaining sources of revenue have fluctuated less, together claiming between 0.5 and 1.0 percent of GDP since 1950 and standing near the bottom of that range in 2008.