The State Department would continue processing foreign applications for visas and U.S. applications for passports, since fees are collected to finance those services. Embassies and consulates overseas would continue to provide services to American citizens.
Social Security and Medicare benefits would keep coming, but there could be delays in processing new disability applications. Unemployment benefits would still go out.
Federal courts would continue operating normally for about 10 business days after the start of a shutdown, roughly until the middle of October. If the shutdown continues, the judiciary would have to begin furloughs of employees whose work is not considered essential. But cases would continue to be heard.
Deliveries would continue as usual since the U.S. Postal Service receives no tax dollars for day-to-day operations. It relies on income from stamps and other postal fees to keep running.
All national parks from Acadia National Park in Maine to Yosemite National Park in California would be closed, as would the Smithsonian museums, including the National Zoo in Washington. Visitors using overnight campgrounds or other park facilities would be given 48 hours to make alternate arrangements and leave the park. Among the visitor centers that would be closed: the Statue of Liberty and Ellis Island in New York, Independence Hall in Philadelphia, Alcatraz Island near San Francisco and the Washington Monument.
New patients would not be accepted into clinical research at the National Institutes of Health, but current patients would continue to receive care. Medical research at the NIH would be disrupted and some studies would be delayed. The Centers for Disease Control and Prevention would be severely limited in spotting or investigating disease outbreaks, from flu to that mysterious MERS virus from the Middle East.
The Food and Drug Administration would handle high-risk recalls suspend most routine safety inspections. Federal meat inspections would be expected to proceed as usual.
A small number of Head Start programs, about 20 out of 1,600 nationally, would feel the impact right away. The federal Administration for Children and Families says grants expiring around Oct. 1 would not be renewed. Over time more programs would be affected. Several of the Head Start programs that would immediately feel the pinch are in Florida. It's unclear if they would continue serving children.
The Special Supplemental Nutrition Program for Women, Infants and Children, known as WIC, could shut down. The program provides supplemental food, health care referrals and nutrition education for pregnant women, mothers and their children.
School lunches and breakfasts would continue to be served, and food stamps, known as the Supplemental Nutrition Assistance Program, or SNAP, would continue to be distributed. But several smaller feeding programs would not have the money to operate.
(Read more: Obama to Congress: Do not shut down the economy)
Americans would still have to pay their taxes and file federal tax returns, but the Internal Revenue Service says it would suspend all audits. Got questions? Sorry, the IRS says taxpayer services, including toll-free help lines, would be shut as well.
Many low-to-moderate incomes borrowers and first-time homebuyers seeking government-backed mortgages could face delays during the shutdown. The Federal Housing Administration, which guarantees about 30 percent of home mortgages, wouldn't underwrite or approve any new loans during the shutdown. Action on government-backed loans to small businesses would be suspended.