Once again, you've sent your tax returns off to the federal and state governments. Or if you haven't, you'd better get down to it, because the deadline is midnight tonight (Monday). In either case, consider this: 45 percent of American households will pay no federal income tax this year, according to the Tax Policy Institute. That's because of the many deductions, exemptions and credits built into the U.S. tax code. Most of these benefit either the very rich or the very poor.