Does anyone know the amount of income a person can earn in the U.S. without having to pay income tax on it?

If single the standard deduction plus the exemption amount.

For 2007 this was $5,350 + $3,400 = $8,750

There are too many variables for a complete answer. For example, if DJ's single person had a child, she could file as head of household, thereby increasing her standard deduction to $7,850 and adding a dependency exemption of $3,400. If the kid was in daycare and the taxpayer's employer offered a flexible spending account or some other reimbursement plan, She could earn another $5,000 (iirc) tax free. The $1,000 child tax credit would allow for another $9,999 of earnings free of any net tax after credits. That brings the total to $29,649. But it's actually higher than that, because she would $578 in EITC. So the actually maximum for a single mom would be higher.

For a married couple, both working, with 2 kids under 17 in daycare, the figure exceeds $50,000 if the right tax deferral and tax avoidance plans are available to the taxpayers.

It depends on how the income was earned, and the person's filing status.

If a person earns $400 or more from self-employment, then they have to file a tax return. They might or might not owe any income tax, but will owe self-employment tax for social security and medicare. If a person earns money where social security and medicare isn't withheld, this $400 limit applies.

For income from a job where ss and medicare are withheld, a single person who is NOT a dependent didn't have to file for 2007 if he or she made under $8740 - for someone who IS a dependent, the limit was $5350. The numbers go up a little every year. There are other limits for other filing statuses.

If you are talking about paying income taxes, as opposed to the requirements for filing a tax return, Steve B. has hit the nail on the head.

see page 2 of attached publication 501

If you are a non-resident, it's only $3500 in 2008--NRAs do not get a standard deduction.