You can travel with only a valid passport to England, France, Germany, Spain, Italy, Japan, Scandinavia, Poland, and many other countries. Most countries require that your passport have at least six months of validity from the time of your entry into the country.

Certain countries such as China, Russia, Brazil, India and others require you to secure a visa prior to entering their country. Visas are issued in the U.S. by embassies or consulates of the country you are traveling. Each country that requires a visa for entry has its own rules, fees and application forms.

Allow sufficient time for processing your visa application, especially if you are applying by mail. Note: that some countries including China do not allow applications by mail. It is the travelers responsibility to obtain visas from the appropriate embassy or nearest consulate before departure from the US. As soon as you receive your visa, check to make sure no mistakes were made. Processing and visa fees vary from country to country, and most fees are not refundable.

• JUNE 2009
  The U.S. government will implement the full requirements of the land and sea phase of WHTI.  The proposed rules require most U.S. citizens entering the United States at sea or land ports of entry to have a passport, passport card, or WHTI-compliant document.
  Please Note: Children under age 16 will be able to continue crossing land & sea borders using only a U.S. birth certificate (or naturalization certificate) after the new law takes effect in June. The original birth certificate or a copy may be used. See the Department of Homeland Security's Ready, Set.. Go! for more information on the changing travel requirements. 
  

Don't shell out for one of those passport expediters, call your Member of Congress. They have these things called constituent liaisons, whose whole reason for existence is prodding other executive agencies. This is the number one way to get a passport quickly. Call the district office for your Congress Critter, which is the number that doesn't start with 202. A list of Members of Congress can be found here. Another thing to try is paying the extra $60 at the USPS for expedited passport service. The best thing to do is, of course, apply early, but if for some reason that hasn't happened, getting a hold of a constituent liaison is the way to go